My first though was to machine a thick plate of Plexiglass. Unfortunately, with each additional feature I machine, the odds of botching it go up exponentionally. 50 holes, 50 counterbores, multiple slots.... do the math. Not great odds of success.
Enter in Fused Deposition Modeling (FDM), more commonly referred to as "3D Printing". Essentially slicing a model into very thin layers and printing the plastic one layer at a time. The great thing about FDM is that the part is very functional, yielding almost full strength compared to a molded part. The downside to the process is that surface finish can be a little rough, but that is not critical for my application. The other stumbling block is that you need to have access to a fairly expensive FDM machine, or know somebody who does.
The design phase of this model is almost complete. The front side has the counter-bores for the battery diameter, but the back side is the tricky part. I've made it thick enough to give some protection from accidental contact with terminal plates and electrically live fasteners. The horizontal bars almost give the visual effect of a radiator, which I kinda like. The real thing will be black PC-ABS, so should look like sweet pieces of hardware.