Here's a realistic 3D printable case idea. Larger than I want for a final design, it will be sufficient for a prototype.
It's 4.6" W X 6.2" H X 1.25" D. It would use 3D printed keys as well.
There's still a lot of mechanical design that will have to be done to be able to mount everything properly inside of it. There's dimensioning and tolerances of the various key holes, the display, the battery holder door, and the code module door. There would also be an on/off switch, a DB9F connector and an audio jack on the top.