I liked both Pahchay's Emergency Cash Keychain (https://www.thingiverse.com/thing:2797977) and I_need_a_better_name's Emergency Cash Capsule (https://www.thingiverse.com/thing:3087577). I liked that I_need_a_better_name's was completely closed, and printed it. It worked well (though I broke it as a test). I decided to make one that had thinner wall and did't have a thick bottom. I wasn't sure exactly the dimension that would work, so I made a parametric model. The wall and base thickness, the cavity height, the breakpoint thickness, etc are all easily changable. I did this using FreeCAD and CadQuery (https://github.com/dcowden/cadquery or https://github.com/CadQuery/cadquery). This means it should be easy to adapt to bills of different size (Euro for example). I ended up playing with the dimensions to get one that didn't try to put infill in the walls, but instead had solid walls of the right thickness. It is still plenty strong, but easily snap-able. The model here was printed at 0.2 layer height, 0.4 line thickness and 100% infill. The infill is only at the very top, and it actually prints faster at 100% than it did at lower percentage due to the smoother movements (at least for me).

Edit - I guess I should mention, you need to add a "pause at height" or equivalent in your slicer before the top closes so that you can insert the money.