Put a sit on seat together with left over schedule 3/4 inch 40 and 80 PVC, some PVC fittings, and a seat I bought off of Ebay for $35.00 with free shipping. Very sturdy, not wobbly, and easily removable. I may add a small sheet of plywood or starboard plastic between the seat and frame with zip ties. Doesn't seem to need it but a test run will tell.
The original seat was very low and consisted of nothing more than snap in pad and a back. Too low and uncomfortable for this 61 year old. Made it hard to get out of too!
Here is is the completed frame. Weights 2 lbs. 8 1/2 oz. minus he seat.
Here it is on the kayak.
Two rear legs slide down into scupper holes and are stopped by the 't's.
Two more legs recess into scupper holes directly under the seat with rubber chair leg tips.
The seat has two straps and buckles underneath that snugly fit around the frame. (Picture shows bottom of the seat)
The seat and frame in place:
Total cost was under $50.00 including the seat.