Has anyone ever had experience in making/replicating pieces of armour in alternative materials like fibreglass or vacuum-formed plastic? I'm curious about how difficult and/or expensive it would be, since there's a fairly big fantasy event coming up next year and I'm looking for a way to infect people there with the historical bug. If a moderate number of such pieces can be made more easily and/or cheaply in nonmetallic materials than in steel/aluminium then it might be worth my time to learn how to build historically-inspired designs in these materials.

There are a fair number of people doing things like this in the SCA, although cutting up barrels is more common as a cheap method. There are a few people doing some serious vacuum-forming though. If you haven't already, you should ask on the armour archives (I check here first in the morning, and then there next). I know there's a guy who goes by "Sir Haroun" who vacuum-forms bazubands and stuff.

I think that there could be a good market for this for HEMA folks, especially since many organizations ban the wearing of metal armour. Most of those groups use straight up sports equipment instead, but I think that there's room for a middle ground, if you can overcome the bias against plastic that is mostly a result of the cut-up-barrel armour, which often looks bad.