The only way that I can see that would be a permanent solution would be to add at least three pieces of steel angle iron at least 1-1/2" on each leg. You would need to anchor them with 3/16" minimum carraige bolts coming down from the top of the platform to eliminate any protusions on top. This would not only straighten the plywood, but keep it straight. Wood, because of its grain, would most likely end up mimicing the plywood slab, and you would still have a curved slab after a period of time.
Daniel & Donna Leonard
2011 Ram 4500 SLT 4x4