Yeah, you're right, SofaKing01 - fiberglass is probably the way to go for the pads.
Inflatable pads would require a bit of annoying R&D, and still not be as durable. Plus, airing them up, etc. etc. Probably, not the best route.
Since the pads look like self-skinning poly-foam (like Nerf footballs, etc.) that's what always comes to mind looking at them in the movie, but playing devil's advocate brings up a couple of issues with foam:
1. durability - once it tears, it has a tear until a new one is made.
2. out-gasing, or off-gasing - depending on the materials chosen, foam pads could stink up the tunnel for quite some time
3. deterioration
Even with a thick, flexible skin (latex, etc.) laid in the mold before the foam backing, the pads could still have these issues. (especially, the smells)
At least fiberglass shells can avoid all of these - it's super durable, no out-gasing once cured & painted and relatively no deterioration)
other advantages: Simpler process - They can be churned out more efficiently...paint them to match, and they're relatively worry free. Less weight, too.
Rog, you're right about throwing some pigment into the resin to save time and paint. Plus, any scratches they acquire over time wouldn't show a garish, original-material-color underneath the paint. Again, rather worry free.
Now, we just have to figure out how to get you a big rig to haul everything you fab up in that new work space, ha!
Nice work!