Your correct about the Pelican 100 not having a lot of room for a cooler... however, I think that I have a solution for what I think you want to do and would work for your particular kayak. Its one of those fish cooler bags. Many of the salty ocean fella's use 'em for beyond the breakers and such. This particular one is only 25 inches long even though you can get them in almost any length you want. Since you have a "sit in" kayak, you could either place it inside towards the front where the access panel would be between your legs or the more traditional way of strapping it to the top front. It has multiple tie down options. This one came from Academy, but everybody sells 'em that is in the kayak business. I hope it helps or at least gives you some ideas.
Paddle Straight and Prosper
~~~~~~~~~~~~~~~~~~~~~~~~
Old Town Discovery 158 Canoe