remember, with seating, the factory sliding/folding backrest is subjected to the ABYC load and impact test for safety reasons. this is to prevent injuries from someone plopping their backside into a seat and the seat back failing and the substrate or slide failing
This is NOT a simply DIY project unless you have knowledge and background in ABYC compliant seating and structure.
if your fiberglass was designed to have both, then source a folding/sliding assembly cushion from the Chap dealer and bolt it in. Note that Chap was recently sold, so the company is in a transition period, and responses may be slowed
If you intend to do this yourself, you will need to duplicate the design from Chap.
you could do a linked back-to-back setup like the single back-to-back lounger seats, and double it up, or split the cushion as the Chap seat appears to be wider than 32"
because if the seat is wide enough for two people, the test weight is doubled.
doing this you are taking tested and safe designs and simply applying a different cushion and upholstery to it.
testing diagram from ABYC H-31
for liability reasons, that is as far as I can point you.
