I spent the week hunting down the best quality and price I could find on plywood for the frame. In my case, I was able to find $39.00 sheets of 4'x8'x.75" and 4'x8'x.5" at the Home Depot in Folsom, CA. I ended up with the Birch Ply and it looks like it's really good quality. No warping, no visible voids..etc. So, I made the purchase and had them cut each sheet into two 4'x4' squares that would fit into the back of my Xterra. Time to break out the Senna plans and start cuttin' some wood after work (I hope)!