I used two sets of 6 sheets of 7/16 OSB cut as shown.
2 sheets 4x8
2 sheets 4x6'10"
2 sheets 4x7'5" cut diagonally
I put an outer ring of pressure treated 2x4s, 2 deep on wooden blocks, with wooden blocks and 2x4s inside every 2 feet. screwed down one set of OSB sheets as above, then put a 2nd set on top rotated one side over. Glue would have been more permanent, but I planned on moving it when I built it, so I've only use screws.