I made one for my raft. I traced a template out of cardboard and then cut the ply. I routed the holes and then varithaned the surfaces. I did not add sand or salt as some people suggest. Another idea is to epoxy the rigging holes and then recut them, then varithane with a few more coats.
The BIG drawback is that this "floor" creates a very big entrapment hazard. I had my foot caught under it during a major surf and was able to get it out before we flipped. The rigging to attach said floor also creates more entrapment hazards. Be aware of this if you plan to have young children or unaware friends or family in your boat
Claimer: Someone that makes a claim that they have been there and done that, can do anything you can do better than you. I hate "claimers"