[aerohead]

You might check out local sign shops 'n see if they have some coroplast that they screwed up on a job and haven't discarded it yet.
Some of the spans you'll be working with will require stiffness,so it needs to be structural materials like coro..
I'd recommend that you don't use the Plex for the belly.It's prone to fracture,cracking,then all out failure.And it's no good near the exhaust heat.
Maybe a local metal recycling center has some discarded aluminum sheet you can buy by the pound if you can't find coroplast.Sheet aluminum also makes for good heat-shields to protect a plastic belly pan.
Aluminum angle extrusion might also show up at the recyclers,good for the spats,if I'm understanding you correctly.