Para rubber may be worth a try I think I have seen it there.

Yeah para rubber is damn expensive... I did our boat upholstery a while back
checked out trademe, clearance stores emporiums.....
ended back at para in mt wellington....to cut costs, I purchased off cuts and glued together... and also use some of the old usable foam and glued slabs over those.

Was in at Davids emporium cavandish drv manukau nest to super cheap. couple days ago, they have some good sized thinner softer foam and a few suitable off cuts.. not many suitable off cuts thu.

Material also expensive , ended up with end of the roll of some heavy duty black out backing type curtain material from spotlight in manukau... and thu had doubts as to how well would stand up in the boat, rather pleasantly surprised.

OH yeah, thin foam under covered panels , end of roll foam carpet under lay from one of the places in that car park area opposite the red shed in manuaku

Total cost including cans of ados  ultra sticky whatever glue , and 9/16 box staples  , but not the new sheets of ply for wall panels  under $150