I fitted a 450 x 450 Bomar deck hatch to my RIB, to replace the old ABS one which had got a bit old & tired. Bomar one is a lot tougher - I think it's made from polypropylene. Cost around £50 (new) in a boat jumble. If you want something totally waterproof, you'll have to spend a lot more. IBS had some aluminium ones listed in their catalogue, but for the same size hatch, I think they were around £400