Thanks for the heads up on Pendaliner. I checked over their site but it appears all the liners are mass produced specifically to fit existing late models - nothing custom or oddball sized.
Regarding the ABS idea, it might work for some applications and ought to be much lighter weight than plywood. But it seems to me if you are going to go to all the trouble of making up the plywood form anyhow, why not just cover the plywood with a release gel and then glass it yourself? Once dry, I would think upholstery or outdoor carpet could be glued to whatever surfaces you wanted.
Then again, maybe this is a good place to apply the KISS theory - just build a simple plywood box and glue on a fairly nice felt outdoor carpet. Heavy - but simple.