you could print a working injection mould from PEEK.

don't think anyone is offering that service yet, but given the difference in cost - it will be a thing at some point.

If you had a suitable printer large enough you could print the seat in one or 2 pieces. PEEK would more than do the job.

Now it all depends what you intend to sell the seats for and what kind of profit you expect to make on each one.
The biggest printer I know of, fully capable of peek (and everythig else) is the mini-factory ultra: https://minifactory.fi/3d-printer/

Not knowing what a plastic base pan looks like - can't really make many other suggestions. Got any pictures ?

It's possible you could use one of the more engineering oriented filaments. ninjatech's armadillo comes to mind.
It's a rigid, lightweight polyurethane and the prints are phenomenal - way stronger than injection moulded abs - and any standard printer will print it easily.

So what sort of size are we looking at ?