Hi Frank,
This is a little complicated to explain & I'm revising my thinking as I go along, so bear with me

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Basically, span from near each end of the cross-piece behind the seat, 2x4 or whatever, to the side cleats. My suggestion is to use (2) PVC pipes, probably 3/4". You'd want to keep the pipes inside the gunnels & clear of the tankwell - basically they need to span above the side air cells.
You could insert small D-shackles through the holes in the top of the cleats. Cut/drill-out a notch in the PVC pipe to fit over the shackle. Drill a hole through the pipe perpendicular to the notch so you can then put the pipe over the shackle & secure it with carabiners or wing nuts. Attach the other end of the pipe to the cross-piece as you see fit, maybe drill a hole through the end of the pipe to recieve a hook?
Complicated...