Here's the some of the info you've asked for
It maybe a better solution to design a frame around the cooler box an strike 2 back rest loops either end
( to support back for passenger sitting on original jockey seat and at the other end - passenger sat on the cooler box
4m Avon Searider Rib
A few dimensions to help get a ball park quotation...
My double jockey mould seat is 12 1/2 Inches off the bottom flange , 16 1/2 Inches including seat cushion
I would need to put the usual backrest cushion on this at the bottom up to the grab rail
so 4 flange pieces/2 plates to fix the marineply/foam backrest cushion would be needed.
The 2nd Back rest would need to be free standing as I'm using an heavy duty Cooler box of the same height as the cushioned jockey seat behind the 1st one ( it wouldn't need the grab rail either ) but we might be able to design a triangle 2 legged stand to give extra strength fixing to the SS transom already on the boat.
Hope this info helps