BIPLEX Fluted Polyprop...
an alternative to hardboard and fibre board.
On Dolomites 3mm hardboard is used in a few places, door trim panels, vertical panels adjacent to front footwells
and the back parcel shelf.
Hardboard is useful stuff but does have drawbacks, it is quite heavy, absorbs water moisture and produces dust when cut (with a saw).
BIPLEX Fluted Polyprop https://www.google.co.uk/search?q=Biple ... 93&bih=939
is waterproof, very light and easy to cut (without dust given off).
It is an extruded twin-wall polypropylene sheet that is pretty rigid.
(If necessary it can be greatly reinforced but gluing two sheets together (with their ribs at 90 degrees to each other).)
It is also a suitable choice for a front parcel shelf. Alas it is not very suitable for the side boot trim panels, but could used nonetheless.
3mm thickness is fine for our applications (it is available in 2 - 8mm).