[pmangels17]

With regards to the backing material, I do have some expertise I'd like to share. I spent four summers (plus two high school years) working at a lumberyard, and I build a lot of stuff with wood at home too. Firstly, almost any plywood is going to be way more robust than any dimensional lumber or planking, because there are no grains. If at all possible, buy plywood, you will not regret it. Secondly, though the difference between S2S (sanded on two sides) and sheathing isn't huge, and both will work for your purposes, S2S ply is so much nicer to work with, it doesn't chip the way CDX or sheathing might, and overall it can be heavier and stronger if you get the god stuff.

Also, if you go to your local lumberyard and ask if they'd like to sponsor the team and donate the material:
1. There's a good chance that they will be willing to help you out, because...
2. They probably have scrap cuts leftover from homeowners who came in but didn't want a 4x8 sheet
3. Nobody wants to buy these cutoffs, so they tend to stack up
4. It's probably good stuff, because people (read: contractors) who buy CDX are doing sheeting work and buy in bulk or cut it on site, and homeowners who buy good furniture-grade wood in sizes that fit in their sedans, and leave the cutoffs behind