GRP pitliner help ?

You can buy G4 Pond liner, which is a rubberised paint on solution. But, it will degrade over time.

 

This project is being vastly over complicated.

Make the pit whatever shape you want out of anything you feel will support the shape desired. If it’s concrete, and not fully dry, use your plastic sheeting to cover it.

Buy the cheapest polyester resin you can find, and some low cost glass, it won’t matter what glass you use.

Apply Two layers of glass and then roll on two layers of gel coat, the last coat with wax added.

If you want a smoother finish you can do that with a little more fairing of the surface, it’s not needed though