When you don't have an original to work from, it is often difficult to create a reproduction. There are usually lots of photos available online, but they don't give any indication of the case dimensions. Often I am helped by enthusiasts that own an original and are kind enough to share some measurements and photos with me. 

Fortunately for me there is a robust Ohio Scientific community out there. A great place to start is Dave's OSI Page. This was an invaluable resource when I was creating the 1P (4P) emulator. For the case, Dave himself pointed me at a thread in the OSI Discussion Forum where a participant name Steve Gray posted all of the very measurements I was looking for.  Using Steve's measurements I modeled some case parts.

I will probably laser cut the "inside" side pieces, CNC the "outside" side pieces, and 3D print the case skin.