Ann you could use either 1/8, or 1/4 and... they don't even have to be marine grade. Is the cedar "marine grade" ?

The glues are all waterproof and Marine grade is just better quality - no voids. I've used good Okume and cheap Luan but usually use the birch veneer stuff 1/4" because I like it better. $8 for a 2' x 4'.

pre coat it with epoxy before cutting it out. I just use a fillet on both sides and it's never, never comming out.

this was discussed a week ago - here's a link to installation

A strip bulkhead looks nicer but's it's not as strong - but plenty enough.

http://www.kayakforum.com/cgi-bin/Building/index.cgi/noframes/read/85010