I agree that the foam that ships with the Shearwater doesn't seem like it is
quite right, unmodified, straight out of the box. I never tried it though and
so I can't really say that it couldn't be made to work given a some amount
of creativity. But a Home Depot within a 10 minute drive of my house had a
number of reasonable alternatives that were quite inexpensive. I ended up
using some thinner, narrower, closed foam, weather stripping, I believe
Frost-King. Probably wasn't more that 10 bucks and this was just one of a
number of customizations/minor modifications that I made during the build
so it didn't seem like much of a problem to me. Hope you can find something
you like. Perhaps you live in an area where it is more difficult to obtain
this sort of material, but it is pretty commonly available around these here
parts.

I think a nicer, more durable gasket can likely be made from minicell foam
or a yoga mat as has been suggested by others. But the weatherstripping
is cheap, easy to apply and easy to replace so I haven't bothered with
fabricating anything nicer than that yet. I'll probably do that though
eventually.

I have found CLC to be incredibly helpful and I do think it would be worth
talking to them about this. They may have some useful suggestions. And it
doesn't seem unreasonable to suggest that they investigate shipping a
different kind of foam in their Shearwater kits. The foam they ship works
well on a Chesapeake, and likely on any design that does not have flush
hatches but doesn't seem optimal for the Shearwater to me. On the other hand,
they may have a good reason for shipping the foam they do and it would be
interesting to know that too.

--
Ogata
eric