Carbon is highly translucent for X-Rays. You will not see it on a photo. Also in MRI scanners (magnetic) it does not cause any problems.
I am surprised that carbon reflects radar so well. There are many stories of small airplanes not visible due to their active radar reflector not working. These planes might have been glass instead of carbon.
Glass is often part of the construction of the radar antenna, the part that says: do not paint. Do you know the visby class from sweden Herman? Made out of carbon, to keep it more stealth. I believe the F117 had a graphite coating, I’ve read somewhere.
(the dutch don’t need that stuff, we have Thales. Active electronic radar blocking ;))
I made a mold from the inside of an Iphone cover, I made some flanks from sheet-metal whit a in the middle so big as the display of your Iphone. I cut the sheet-metal plate in half and put them together whit tape and fit the cover ride over the hole in the sheet-metal
Soon I whil make a new one whit more clearance between the flanks and the fake Iphone.
I hope my story makes sense, or els you have to wait till i make a new one 
Gr Henk
This may be a dumb question, but I keep hearing that carbon fiber doesn’t work as a case for the iPhone because it blocks signal. Is this the same for other phones too or will a carbon case work for them since they don’t have alum around the outside?
It will not work on any phone. There are plenty of phones with at least half the case of metal.
If I had an iphone, I would probable make a 2-piece mould from the original (metal) cover, then replicate it in glass, perhaps with some fancy strand of other material, or wood veneer, or printed glass veil (with wood print), or anything other fun. (colour copy of a 100 dollar bill or an old fashioned pcb). You name it.