Here's a copy of a post from a while back:
Drop Seat Mount
There was a thread earlier on ways to mount a seat (over a cooler, etc.) Here are some pics of a flip-up seat mount I just built for one of my homebuilt frames. The previous mount was a 1" board that rested on top off the crossbars, with u-bolts front and a strap on the rear.
The problem was that when I raised the oars out of the water they'd hit my knees. I also wanted to lower my center of gravity a bit. So I wanted to drop the seat as much as possible while still being able to raise the mount to rig gear underneath.
It's thin steel plate, bent in a vise, with aluminum angle (painted) riveted along the back and u-bolts (cut down) front. To stiffen it, I riveted smaller aluminum angle along both sides. The pressboard is both stiffening and a spacer, so the seat clears the crossbars.
This shows it raised.
The seat is a cheap fishing thingie from Cabela's: comfortable, lightweight, and pretty tough (I weigh 225).
This sort of tinkering is fun.
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For a flip bracket to fit over a cooler or drybox, you could reverse the geometry, i.e.
mount the flat part on top with the angle bend and u-bolts down. Not exactly rocket surgery. . .