I don't think you'll find something that will fit your boat, they're usually pretty specific.
Get yourself some camp-mat, maybe some minicell foam and some glue, make that thing nice plush.
something like this
you can maybe add more or less to certain areas to make it more comfortable.
Alternatively you could go with leighton's idea. You could rip the bottom off, stick the castors on the outside of the boat, stick the seaty thing in the seat place and shove the remaining upright thing up your hole - you now have a craft suitable for any fun adventur that you can think of.
Regards,
Niall