It is simple to make one using the same methods for making dry bags in: http://www.paddlewise.net/topics/boatequip/drybag.pdf
or
http://zaphod.iar.nrc.ca/~mbastian/drybag/

These methods have the advantage of being totally water proof when the cockpit fills up. I am currently about to make mark 3 version. 1 was to small and 2 is over large. Mounted on runners and attached with snap locks it is easy to pull out and remove. Mark 2 version even has shoulder straps to use as a day pack. Leave an outside seam of about 1cm on the bag and you can sew on as much webbing or attachments as you desire. Another idea is to use elastic straps running across the bag (the top where it touches the deck) so that they reduce the size of it when the full volume is not needed. Fun to design and make, yet one of the most usefull accessories I have.