Re: Composites radio transparent??
Some composites are radio transparent, some are not. Same with resins, some are, and some aren't radio transparent.
If you don't have access to the spec sheets there are lots of ways to test for radio transparency, but they're complicated and difficult to carry out at home/secret bunker. If you're in doubt just check across the material for continuity with your multimeter. If you've got continuity you've got all sorts of options from there, but at least you've got a good place to start.
Incidentally, if your composite material is conductive, you need to go back and assess your wiring paths with an eye for situations that can create a ground loop. Things like power and signal wires (anything with much of a voltage differential really) running parallel or very close together or touching are potentially (Ha!) bad. If high differential wires have to cross you want them at 90 degree angles angles at the intersection.
I don't know why you would need more than one ground in such a small place, but maybe you have reasons. Minimize the number of ground points as well and make certain they are absolutely immobile. A dab of CA never hurt a spaceplane you know.
Saying the following is probably unnecessary, but I'll say it anyway. If your material is conductive but your resin isn't and you run a screw through the resin, and into the material, you've just created an antenna. Same with your antenna lead if it pierces the material.
All that to say it's definitely worth testing your materials and making adjustments to internal systems to reflect your findings. If you create a serious enough ground loop it will seriously screw with your signals in direct proportion to the electronics draw, which is almost assuredly going to be when you've attained apogee (because that's how the universe works).