I've had a big pile of quarters on my desk at work for a long time, and what better place to stash them than in this nice little treasure box? The slot on top is sized for a standard quarter.
If you're willing to commit to saving up for a rainy day, print it as a single object, and when it's full you can smash it open. If you'd prefer to keep it re-openable, there are holes in the back and front that are designed to be threaded with zip ties for a semi-permanent seal.
The OpenSCAD file attached is parametric - you can adjust the dimensions and the wall thickness by using the parameters on top.
There's a lot of room to add more surface detail to this model if someone wants to jump in with some derivatives. Also, I'd love to design a version that has snap-together hinges, or better yet, the dovetail-groove style connection like in thingiverse.com/thing:11977 for a nice, snug closure.