Acrylic sheet is cheap, easy to work with and comes in various colours.
If you use GRP, then you'll have the uneven glass surface to counteract. This could be machined smooth, if you were insistent on using a GRP flat section to overlap the existing hole. The edge would need to be finished, eg with flowcoat, so quite a bit of aggro.
Neatest option would be to dress the hole back flush with GRP and make good a new opening for the radio in the fresh glass, but quite a lot of work.
Bigger radio would have been good.