A Youtube user named wermy managed to put RetroPie into an Altoids tin a while ago, but he was reluctant to open it up and show everyone because of all the ugly, visible wires inside. Now he's back with a new & improved version, thanks to 3D printing, and it looks pretty enough for him to show us its guts.
Other nifty extras included in the new version are a hinge to make sure it stays open nicely, a better screen, and WiFi, making it easy to transfer ROMs.
If you're wanting to make a similar thing, wermy promises that a guide is on the way, and he'll make all the necessary files available soon. Obviously for this specific project you'll have to obtain an Altoids tin and access to a 3D printer — for those unfamiliar, Altoids is a US mint (which is why the name of this project is MintyPi), and you can grab the tin from stores that stock US foods, or you can order it online.
Of course, there's nothing saying you can't just put your RetroPie guts into something else. Getting creative is part of the fun! But since wermy's files will be fitted to an Altoids tin, that means jumping into a 3D modelling program and making new CAD files to fit whatever you have.