Material:

I've played around w/ clear silicone seal w/ great results for other applications, but Ian Johnson's point about urethanes having better adhesion than silicones makes me think that Ross's material is prob best. I presume you can get clear or black.

Location:

You want the gasket on the hatch. Otherwise if it is on the flange, it is always exposed to damage when you put the all the crap in the yak - stoves, tents, etc etc. Also if you drop something it'll get damaged. Or sand gets trapped betw the gasket and the deck - hard to clean. If it's on the hatch side - its always protected and so easy to clean - just swish thru the water. It is also so much easier to repair if it is damaged. Just take it with you to work on near the fire - or in the tent - or in the house.

-mike