Here's another homebuilt frame for a Jack's cat. Since Jack doesn't sell frames, I guess you could say he's fostering our creative impulse.
This one's different to the others posted: it's steel tubing (1" IPS, 1.315" OD) with 1-inch SpeedRail joints and 1" conduit elbows, bolted and braised. Cost about half what an aluminum frame does.
This view shows the braised elbows (the drops for the floor).
Here 'tis with the rowlocks, seat, cooler (also the rear seat) and split decks for dog transport.
And here it is on the water: Deso, October, 2008. Extra gear frame up front, for York Packs. Long, skinny, low-rider cat, nice for snaking between rocks.