Instead of giving my nephews some cash in an envelope (which is what they want) I decided to check out Thingiverse for some inspiration of something a bit more fun.
I found this cool locked box and decided to redraw it to make it bigger so I could stuff some cash in there. I also made it a tighter fit and a bit more rounded while I was at it.
The name on the side is not included in the model I uploaded of course.
Please see the video of the original thing to see how it works. It's pretty straight forward.