GRP end caps

Anybody tried using GRP sheet to laminate WTC end caps?

Whilst building a prototype layout for WTC I used perspex but both glues (superglue & gorilla glue) I tried failed to bond, so I'm thinking of using GRP sheet to build the inner frames and the end caps using an epoxy glue.
2 x 5mm disks could form the inner part of the end caps with room for 2 O-rings and a 3rd oversize as the outer. I can turn the disks on a lath to get the close fit.
I am intending to use 75mm polycrbonate tube for the outer tube, as polycarbonate seem to be prefered over acrylic.

The only down side I can think of is that I have heard GRP can absorb water and because of the fibres the surface mght not be as smooth as resin, perspex or similar materials.

Again just my personal views.
I wouldn't give up on polycarbonate, just yet. It is wonderfully tough but machinable. You can glue it with the usual solvent adhesives e.g Plastic Weld or two part adhesives like Devcon plastic welder (This is also very good for PVC)
I have never liked Perspex - brittle and badly affected by things like superglue and even Loctite. (I would keep these things away from polycarbonate too.)
I have never used anything other than polycarbonate for end-caps but I think that GRP sheet could give leakage problems through the laminate? (3D printed end caps have a similar problem - a lathe still has it's uses!)

David