Just tagging along here.
A bunch of us put together a couple of platforms this past weekend. Ours' ended up being 8'x8', with a 42" centerpiece. We fortunately had 4 mats laying around, so were able to devote two per platform. Each mat was cut into two 2'3"x4' sections (plus scraps). We used AB plywood for the centerpiece and CDX for the base layers.