Also found on rowing shells, so you can source through those companies:

http://www.rowalden.com/product.php?cPath=2_73&products_id=201&osCsid=c28f6597ab46b4199dc8bd38ca89f22a

I've also seen one at a local West Marine - but only once in multiple visits to many stores (we have a lot of boat stores in Ft Lauderdale areas - go figure!).

These are made for single wall/shell installation. About all you'd have to do on a Mystery is cut the hole and install - and then remember to keep it closed except for short periods to drain while moving. You'd be reminded pretty quickly if you forgot!

I have been thinking about these for sometime now and might incorporate one if I ever get around to building a stripper. Seeing one on the Mohican makes me think it's not so crazy - but then again Mohican is targeted as a flat water boat, and shells don't typically get too far from shore either... Not sure I want a hole on my hull that relies on a rubber seal and a mechanical latch if heading offshore.