To anyone interested, my platform came out pretty nice.
It ended up being 6'x8' with a 3.5' center cut-out. I used OSB for the base of it. The annoying part was that the OSB had some weird blue spraypaint on the edges that was getting all over the place, so I wrapped the whole thing in plastic before putting the nice piece of birch and stall mats on top.
I used a polyurethane/stain 2-in-1, and it doesn't seem to be too slick (some people warned me).
The thing has got to weigh 400lbs... I cannot move it without using a wall to squat the damn thing.