Jim:

When I faced the same question with my Expedition Single, I went to my local kayak shop and was told to measure the circumference of the cockpit. I had enlarged mine and found the size was 84". The shape was basically the same, so all I had to do was to find one that size. A couple of notes.. I was looking for a lighter weight model like nylon instead of neoprene. This meant I had more options in spray decks, since I didn't need a drumlike fit. I also found a reference card at the store that listed my cockpit circumference as "medium", which gave me an idea of what to shop for.

Ironically, what actually happened is that I wound up buying a second at an outlet store without the shock cord for $1.00. I installed my own cord and use it all the time now. It is slightly larger than my cockpit, which is nice because I can move around a little without worrying about pulling it off the rim. A little gathering or bunching of the fabric that doesn't compromise the seal is just fine.

Sorry you've had bad luck with the stores so far. Here's a link to the store where I got the most help. Good luck.

http://www.nwoc.com/

Ed Valley