My two cents and questions since I have been looking at this also, and may
tackle this job this very weekend, don't know.

The original plywood panels are a 3-ply 3mm thick panel, which makes it just
under 1/8" in thickness.  The 1/8" would be an ok substitute I think
regardless of the material (mdf, plywood, etc.)  1/8" plywood is available
but harder to get in some places.

I have seen at the home improvement store(s), some plastic sheet material
used for shower stalls, etc. and it seems to be less than 1/8" thick.  Has
anyone used this sort of material?  It could be painted or covered with
vinyl, but it would never delaminate like mine has.

Another possibility is to rehab the existing by using epoxy and clamping
it.  That's what I was going to try first.
--
Alan Felder
Austin TX
82 Diesel Westy