These are not very commonly discussed around here at all, if ever!
(To my memory!)
Do you have any examples of commercially available, ready-made, fiberglass radar arches?
Many production boats use Fiberglass/polyester radar arches. But the tooling for one would be relitatively expensive. Many of them are actually aluminum, with paint--rather than glass. The arch has to be hollow, for the wiring (and weight).
It certainly could be done--but unless you want to make your own molds, It would be expensive. My Radar mount is PVC pipe--and I have done 4 Radars that way, and it works well. (and cheap).
Fiberglass arches are ususally used to blend the arch into the overall boat design. Since the C-Dory "look" isn't about sleek and sexy, I would think a fiberglass arch would look odd on the boat.
It would certainly be interesting to see one done, though.
I have a call out to fiberglass shop to see if it can be done locally. There are a few images on the internets but none that really strike me as proper for a C-Dory yet.
If I can't get it done locally, I probably won't do it at all and just stick with a bolton stainless radar arch.
Are you going to glass a fiberglass radar arch onto the C Dory roof? If so, consider how your are going to do that. To make it blend in, will not be easy or cheap. If you are having a fiberglass shop build an arch, then I assume that you would have them also apply it to the boat. You will still have to deal with a hollow structure, for wiring. You will have to have access into the center of the arch.
Just an update, couldn't find a willing fiberglass expert to make an arch. No worries, went with a nice stainless. Sometimes money won't buy everything. :?
I haven't seen any of this in a long time but they do make fiberglass electrical conduit. It's often times used on cooling towers and smoke stacks. What I installed many years ago was 1 1/4" trade size. It might be handy for some applications around a boat.