A few years ago, I created an acryllic piggybank for myself, but it needed feet.. so I whipped these up in OpenSCAD.
These can be used for other boxes too (shown in the gallery).
The code is old. and I think it can be optimized as it takes quite a while to render, even with $fn set to 30, but hey.. there it is :)
Infill can be whatever - more than 10 is probably ok, but experiment. I'm doing mine wall thickness 0.6mm and 20% infill.