I started with a 4/4 thermoplastic violin case I purchased from http://www.musiciansfriend.com
Step 1: Initial Case Interior
The case comes lined and custom fitted for a violin. The material is a type of faux fur that is somewhat elastic. The base is styrofoam.
Step 2: Remove Interior
The lining material is glued to the styrofoam with spray adhesive and and styrofoam is glued in with hot melt glue. It takes a bit to get it started, but after you get under it, it pops out rather easy. A large flat screwdriver or small flat bar would be well suited for this. Don't worry about busting it up or tearing it. It'll all be throw out and replaced.
The only special note is to ensure you get almost all the hot melt glue out. It is an absolute bear to pull out in some places, but it all has to come out. I left a small bit in my first case and it came back to haunt me when I went to install the retention straps and the final lining.
Step 3: Prep for New Base
Once all the old lining is removed ( I know, I know, I didn't get all the glue in this one. It was a major pain later) tape off all the aluminum edges on the interior with painters tape.
The tape will prevent any of the expanding foam from sticking to the aluminum. If it does get on the aluminum it has to be scratched off, marring the finish.
The blue tape will also give a guide as to where to fill the case with expanding foam.
Step 4: Add Expanding Foam
Start spraying the expanding foam into the case trying to stay just barely under the tape line. Once the foam expands and cures it will rise above the tape in places, but it can be trimmed down. One can of foam got me about 75% the way through. I had to make a mad dash to the hardware store to get another and the first section had already started to cure. The remaining foam didn't quite adhere as I had hoped it would, so, lesson learned. Have 2 cans ready.
Step 5: Manage the Expansion
The foam tends to mound up in the middle and stay thin on the outer edge. To manage this a bit and force the expansion to the edge, I put a sheet of newspaper over the foam after it started to form a "skin". I then laid a couple weights on it to push in down in the middle. Nothing super heavy that would form a crater. Just enough to level it out a bit.
Then it's a matter of time for the foam to cure. I left mine for 48 hours but I think the can says it's cured after 24. Read the can.
Step 6: Leveling Out the Foam
After your cure time, you have to level out the foam. I used a 12" hacksaw blade which really worked out well. I took off all the really tall bits and then started bringing the top down so the foam is about 1/4"-1/2" below the aluminum edge. I kept the shop-vac handy to get all the little bits of foam.
As soon as I cut into the tallest part of the foam, I found an area where air pockets had formed.
I simply refilled that are with more foam and allowed it to cure before continuing.
Step 7: Laying Out the Interior
Now it's time to fit all your stuff into this box.
Lay in out and then mark around all your pieces with a marker. The magazines in these photos will be double stacked.
Try to keep in mind where you are going to be adding the retention straps and where they will be anchored to the case.
Step 8: Get Trimming
First things first, take the hacksaw blade and cut straight down through the foam all the way to the outer plastic shell. Don't worry about the pieces coming out. The foam sticks like the devil to the plastic. Plus the whole thing will be wrapped and glued to fabric in the end.
Once all the outlines are done, put a slight bend on each end of the hacksaw blade, about 2"-2 1/2" from each end.
Use the bend blade to make scoop cuts inside the outlines to remove small pieces at a time.
Step 9: Finish Foam Trimming
Periodically place the items you're cutting out for into the foam. Remember, there will be a fabric liner, so don't make it super tight.
The biggest concern is getting the depth right. Taking a bit at a time is best because if you go too deep, it'll require trying to glue your cutouts back in.
For the openings that get down to the outer shell or close to it, I took a bit of the original liner and glued it in. It acts as an additional cushion and prevents any banging sounds while carrying the case.
Step 10: Adding the Retention Straps
The straps I used are 3/8" velcro which has one fuzzy side and one adhesive side. It comes in rolls and is commonly used by guys setting up comm closets to bundle server and telephone wires.
I used 1/8" aluminum pop rivets to secure the straps. Right next to the rivets in the hardware store, there were small 1/8" aluminum washers that give a little more grip to the rivets.
I cut the foam back and removed a small plug where the strap is to be installed. Drill a hole through the case shell, and the strap.
Push the rivet through from the outside, slide the strap over the rivet and add a washer.
Run the straps long, you can always cut them back later.
(Make sure the soft side of the velcro is facing your gear. Don't want the rough side scratching up your stuff)
Anchor the rivet down and glue the foam plug you removed back in. (Hot glue, spray adhesive or good ole' Elmer's will do)
Step 11: Pre-fit the Fabric Liner
At your local fabric store, pick out the pattern and texture that you like.
It helps if the fabric is a little elastic. It helps when getting it into the tight spaces.
I really liked the black faux fur and got a yard of similar material. I think it only ran me about $10.
I cut the yard in half, draped it over the case and started tucking it in as best as I could. I used the rifle and magazines to help hold everything tight as I went. Be mindful of the edges. You want to be at least 2" over the edge, all the way around to make sure you have enough to finish the case.
Once I got it where I wanted it, I cut small holes in the fabric to allow the retention straps through.
Step 12: Glue in the Liner
WEAR RUBBER GLOVES.
Once you start fiddling with the spray adhesive and the fabric, you're gonna be 3 different kinds of sticky.
Step 13: Complete Liner
Pull up a portion of the fabric to expose the foam.
I did mine one third at a time.
Spray both the underside of the fabric and the foam with spray contact adhesive.
Allow it to get tacky and then work it into place.
YOU'RE ONLY GONNA GET ONE CHANCE TO GET THIS RIGHT.
Once the adhesive touches, it's a pain to remove and re-position.
Once it's in, use your gear to hold it tightly in place. Strap it down tight and be careful not to get the adhesive on your equipment.
After it's all secured, trim the excess fabric so it extends about 1/2" over the edge.
Then, using a small screwdriver or blunt end, tuck the edge of the fabric down between the foam and the shell.
Step 14: Done
Once everything is dry, it's just a matter of trimming the retention straps to the correct length.
The AR-15 case was my second case.
My first one was for my MP5. This one did not have retention straps (big mistake) and two compartments that were supposed to stay closed with magnets embedded in the foam (didn't work and resorted to velcro)
Overall I think they are pretty big successes.
Let me know what you think. Feel free to ask questions or make suggestions for improvements.