I would lay up a patch of the OEM resin type, several inches greater in diameter with a similar thickness (and core
if any) on a sheet of waxed plexiglas. Taper both the deck
to fit flush... But just a wee bit lower. Bond with thickened epoxy, but be careful not to over do it with too much epoxy. After cure, fair, then coat with gel coat or whatever your current
top coat is. If gel coat, you might be able to replicate the non-skid pattern by taking a mold
off another section with silicone, then impressing it into the wet gel coat. REMEMBER YOUR PARTING COMPOUND!