The Ikea Busktoffel shade looks nice, works well, is relatively cheap, and comes in several sizes. If you are lucky, one of the sizes fits your window. If you are not lucky, or if you are looking for a really custom fit, then this instructable can help you trim the shade to the exact size.
The busktoffel is not hard to customize because it is mainly aluminum and paper. Also, it is not hard to disassemble. You will need a way to cut alluminum. I used a miter saw with a aluminum saw blade, but a hack saw with a metal saw blade will suffice.
Hint: If you have more than one Busktoffels to customize, it is a good idea to leave one un-disassembled so that there is a reference on how to put it back together. Otherwise take pictures before disassembling if you are not sure. If a part gets re-installed backwards, just take it apart and reassemble again.
Step 1: Determine How Much You Need to Cut Off.
It is easier to figure out how much you need to cut off than how much you need to keep. Test fit the shade to the window and measure how much excess length it has. The excess length is what you need to cut off. If you are using a circular saw to cut the aluminum, don't forget to account for the thickness of the blade.
Take note of how the shade is oriented on the window. There is a front side and a back side.
Step 2: Remove the Pull-down Bar.
Remove the pull-down bar. The bar is aluminum, but the end caps are just shiny plastic made to look like chrome. The caps are friction fitted so it can be pulled out or tapped out with a screwdriver/block of wood/etc. I made a special tool to knock the cap out but that is not necessary and probably a little overkill.
You just need to remove one cap.
Slide out the pull-down bar, but pay attention to how the shade is inserted into the pull-down bar before you slide it out. This is so you can reinsert the bar back later. Take a picture if that helps.
Step 3: Remove the Top Bar.
Remove just one of the brackets that holds the roll of shade to the top bar (one small philip screw). Only one side needs to be removed.
Disengage the roll of shade from the top bar.
Note: The roller mechanism might start to unwind. It is OK. Let it unwind.
Step 4: Trim the Top Bar and the Pull-down Bar.
Cut out the excess length from both the top bar and the pull-down bar. The excess lengths for both of them should be the same. Keep in mind that all excess lengths are exactly the same. This means all excess lengths for the three long aluminum pieces as well as the paper shade to be removed are the same, but the lengths you want to keep are not necessarily the same.
Note: If you are thinking of using the first excess piece to measure how much to cut off the other pieces--don't forget to account for the thickness of the blade.
Step 5: Remove the Tensioner Mechanism.
The tensioner mechanism is on the left side of the shade.
Peel back the shade from the white outer ring of the tensioner. The shade is adhered using a double sided tape. I use a blade to cut/scrape the tape off carefully. It is OK to tear up the double sided tape--there should be enough left to re-adhere the shade paper back to the aluminum roller. However, be careful to not mess up the shade paper too much.
Once the shade is peeled back from the white ring, use a flat screw driver to pry out the tensioner. Do this slowly.
Caution: The tensioner is probably under tension so it will want to unwind once disengaged from the roller tube. Let it unwind but do it slowly and avoid injury.
When the tensioner is done unwinding, pull it out completely.
Step 6: Trim the Shade Paper.
Trim the same amount off the shade as the pull-down bar and top bar. A pair of scissors works.
I used a sharp blade on a cutting surface that has a groove. Whatever works for you.
Trim it all the way, including the shade that is adhered to the roller bar (you probably have to use a sharp blade there since scissors won't work) and the translucent plastic rib stapled to the edge of the shade.
You may need to put in a staple or two to secure the end of the plastic rib to the shade.
Step 7: Reinsert the Pull-down Bar.
Slide the pull-down bar back into the shade. Insert the end cap by pushing it in or gently knocking it in.
Step 8: Expose the Roller Bar Cutting Area.
Before you can cut the roller bar, you have to peel off the shade paper. The area to peel off is on the longer side of the bar, i.e., on the part you want to keep. Peel back about 1.5 inches, or whatever amount you need to make it possible to cut the roller bar.
Step 9: Trim the Roller Bar.
Mark off the roller bar and cut. It should be the same amount that you cut off from the pull-down bar, top bar, and shade paper. You should measure from the end of the aluminum roller bar--not from the edge of shade paper, nor from the plastic end piece (which should have been removed by now).
For all my aluminum cutting I used a non-ferrous metal blade on a miter saw. But a hack saw will also work with a little more effort.
Step 10: Reinsert the Tensioner.
Put the tensioner back in. There are grooves on the tensioner that should coincide with the key on the roller bar.
Push it in fully.
Attach the shade paper around the white tensioner ring as best as you can. There should be sufficient sticky tape material left for this. It doesn't have to be perfect because it is unlikely to be seen in normal usage.
Step 11: Retension the Tensioner.
Roll the shade completely onto the roller bar (do this without attaching the top bar).
With the rolled shade, attach the top bar, and then reinsert the side bracket. You do not have to put the philip screw in the bracket yet. But make sure the roller bar's keyed ends are properly seated in the slots of both the side brackets.
Now carefully pull the shade out/down fully. Make sure that when it is fully out, the tensioner is "locked", i.e., it is not trying to roll back up. This step will wind the spring in the tensioner.
However, this is not enough tension on the spring. You will have to repeat the steps again:
Carefully detach the bracket again (that is why you don't put the screw in yet), and then remove the roller bar from the top bar. The shade is fully pulled out at this time.
Roll the shade completely onto the roller bar again.
Reattach the top bar and bracket, again.
Test for sufficient tension by pulling the shade down and see if it can completely retracts. If it is OK, put in the philip screw and you are done. Otherwise pull down the shade a little bit and repeat the process.
Note: The top bar has a front side and a back side. The groove side is the back side, which faces the window.