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This is great. No one would steal my old desktop computer, since a hock shop wouldn't give them anything for it. Might even put a sticky note on it indicating it has a bad mother board or something similar to make it seem worthless.
There are only a couple here that are "safe" - meaning someone won't toss them out, along with your cash. The sprinkler head is one, and the electrical outlet (if you install a special outlet box that is, like the tardis, :"bigger on the inside". Those are available at your local building supply, and can be rigged so that even if someone removes the plate and outlet, your hidden items are still not visible.
Good way to hide something, but don't use this for a Pi 3. The insulation provided by the book pages along with lack of ventilation will burn the Pi. Burnt Pi isn't good.
You can get a box that's like the Tardis - bigger on the inside, which has another advantage: it can appear empty even if the plate is removed. Your money or microSD card can be taped "around the corner" and out of sight.http://www.homedepot.com/p/Carlon-1-Gang-17-cu-in-Shallow-Old-Work-Box-B117RSWR/202077341
Cool. Just remember to NEVER wear it on a job site. Besides the laughter, the switches make a perfect way for high voltage to get to your head from any wire you might brush up against. OSHA would have a fit.