Hatch cover for a 1999 OT LOON-13

I have a lady-friend-paddler who paddles an Old Town Loon-13 kayak (1999). I would like to give her a hatch cover so she no longer has to slide her seat forward and back to stuff gear in the stern. All that sand is messing with the slides.
The opening is 16" long x 11" by 8".
I tried Top Kayaker and Old Town and, of course, Google. No luck. I may have to make one for her from an old barrel and heat-gun but if I can find one to buy, that would be ok.

I can add straps to keep it from leaving the boat and it does not need to be waterproof as she has no bulkhead. Just something to cover that hole I will cut to make it easier to stuff her dry-bags.