I have an OT Otter with the hard plastic seat. I cut a piece of blue 3/8" foam from a camp pad I bought at Walmart. Glued it on and it helps. I have a Yakpad that I bought to put on my OT Guide 119, just the seat portion. Will let you know how it works. Right now, I am using it as a cushion on my oak desk chair and it feels really good.
PFD's (Life Jackets)
Paddler's Truck Rack