Valley oval replacement hatch cover
I needed to replace my old and cracked hatch cover for my old Valley Avocet ( plastic ) kayak.
The original covers were tough to find so I bought a Sea-Lect Performance cover from Tom's Top Kayaker Shop. ( A great place to find hard to get stuff.) 
I've rolled the boat a few times though I have not been in pounding surf and the cover stayed on and keeps the hatch dry.
It's WAY easier to put on and take off than the original and is made of a different material . Perhaps it has some vinyl in it.
Granted, it hasn't been given the ocean test but so far it’s great.

Along the same lines, the Scorpio that I just bought had the awful KayakSport 8" hatch cover with the stiff surface and thin, smooth sides.  It did nothing to keep the water out of the day hatch in mild conditions, much less rolling or in bumpy water.

Kevin Beckwith turned me on to the Valley 8" hatch cover made for Necky kayaks.  It's the same size and is the legendary Valley construction (flexible with channelled sides that you can put a line around).  Tom's Top Kayaker has them.  Interestingly, they are sold out of the same exact 8" cover without the Necky logo.  So if you can stand having a Necky hatch cover on your P&H kayak, it's the same size and works much better.

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