Interesting idea, but I doubt it would hold up to a "brute force" attack by anyone who realizes what's inside, I've installed my fair share of drywall and I know gypsum is pretty easy to break. I'd suggest instead of marking it with a Bitcoin logo or something similar (as seen on the video), just leave the cube plain with no identifying marks. Better yet, if you can sculpt, make the mold in a different shape to disguise the whole thing as a small decorative statue.
Overall I don't think it's a bad way to store your money long-term as long as you don't make it obvious what's inside. Definitely better than leaving a paper wallet lying around where it could be stolen or photographed.
edit: It would be cool (but pretty obvious that it holds money) to make a mold in the shape of a "piggy bank", bonus points if you can seal the film canister inside while leaving enough hollow space to actually drop coins in through the top.
