Davej wrote:

Anyone know where waterproof zippers can be bought? I might ask a
friend if she could sew one on.

You can add a hatch cover like a Beckson in small diameter. Cheaper than a
waterproof zipper. I think there were instructions in Sea Kayaker magazine some
time back. You need to make a ring to attach to the inside and then bolt the
hatch flange to the ring from the top with sealant in between.

Mike