Thanks for the info. What I did was take the stl files and converted them to step files (and a solid model) using Freecad. Then I brought it into Alibre and worked with it, creating an identical model (or nearly identical) in Alibre. The converted STEP model was terrible, and some of the parts were difficult to work with (couldn't even measure them), but I did create a case for my PI (and printed it) using Alibre features.
I did need the calipers because I couldn't take some measurements directly from the converted STEP model.
I'm getting to like Freecad.
Now I can work with the case to modify it to add a buck converter board to step down the voltage from the 24V printer voltage to the 5V the PI needs.
BTW, I have 3 Pi3Bs, so I have the board in hand plus I printed the stl case to see how the Pi fits in it.
Anyway, it's done