To be honest, I'd avoid glassfibre too. It's difficult to cut / drill neatly and you end up with cracks in the gel coat and glass fragments everywhere. I'd consider an aluminium panel spray painted if you want white. Either that or ABS (Antilock Braking System) is nice and workable as you can drill and file it nicely. I've never seen GRP sold just as sheet, as it tends to be used when a shape is required, as it's advantage is it's ease of making any shape you like.