Let's design some nice looking handy casing for Alien Die. 

There's a legend that Palm Pilot creator carried around a piece of wood imitating the device and shaped it to the perfect form with a knife.

I'll make it much simpler. The device will have a grid of buttons  3x3 that's why it will have a square shape. The side of the square is defined by the battery holder's largest dimension which is about 1'' in length. And the same for thickness: battery + PCB + buttons + shell = all in all about 1/2''. And since the battery doesn't occupy all the space under the PCB, there will be a small indent for a finger.

After multiple tryouts of different CAD applications I finally chose Fusion 360. it is powerful, it has a pretty good UI and it's free for non-commercial creators like me. There are plenty of tutorials on this CAD so I quickly fixed a scale model.

You can find STLs here github/alien-die/stls (the final version a little diverges from the picture above):

So it's printed (the bottom piece was printed vertically to make it safely bend along layers and fit in grooves of the main body like a spring):

Parts cleaned of supports (the middle piece is the imitation of PCB):

And finally it's assembled:

Advice #2: Make scale models with all internals, even fake internals. This is how you can assess usability, find out all problems and all mismatches. This is how I knew and fixed the unfit locks, bad dimensions and found out that buttons poking out of the casing was a bad idea. :-)