Standard guitar pedal enclosures are good. Mammoth Electronics used to be a great place for cheap painted seconds, but Small Bear Electronics has a good variety, and has custom colors as well.
I actually ended up putting my Open Theremin in a Ken-A-Vision projector microscope casting! I had to remote-mount the knobs and switches, and I put a power supply in the base (so that it works with a toggle switch) and it has great grounding using the big cast aluminum structure! That alone really stabilized the Open Theremin for playability. It also makes for a nice heavy base, so that it is far less easy to knock over than before!
If you do need to separate the antennas a bit, coaxial cable is the best thing to use. I had to try several different types before finding one that worked well, but it turned out to be one of the ones that is shielded by a foil layer, rather than woven. I was lucky that I had some leftover from a vocoder project.
The other nice thing about using a guitar pedal box is that the aluminum is thick enough that you can drill and tap it to a camera tripod thread, giving you bazillions of mounting options! Such as a SmallRig camera mounting clamp, or any camera tripod.
Good luck!