Are you talking about G10 (Garolite)? I used that for my thru-hulls; also, got it from McMaster-Carr. Used West Systems 404 mixed into epoxy for the adhesive to the hull. I think that is what people used on other threads on this topic.There is a fiberglass product I know as 'strong-back' which I believe is 'structural fiberglass' at McMaster-Carr (mcmaster.com) you can get that and fabricate a backing plate for yourself. You may be able to buy something like you want at McMaster, whether or not fiberglass
Maybe this will be useful: I have used G10 for a number of different things. It is really a hard material. I use a saber saw with fine teeth to rough cut rounded shapes. Then do final shaping using either a sanding disc on the table saw or sanding attachment on the angle grinder. This makes a lot of dust. The dust will coat everything in the shop. I do it outside if using the angle grinder. I rigged up a vacuum attachment for the table saw.
When cutting G-10 on the table saw, I buy cheap plywood blades (fine spaced teeth). They don't last too long. But normally, there are not very many cuts to make.