This is what I used to build my platform. Here’s what I did:
2 sheets of 4’ x 8’ Plywood side by side (running east and west)
2 more full sheets side by side running north and south on top of the the first two. This keeps the seams from lining up.
1 higher quality 4 x 8’ sheet of Plywood. You can leave it at full size but I ripped it down a bit so there is more room for the plates on each side. I think I cut off 6 inches so that it’s 3.5’ wide. This piece obviously goes in the center of your platform.
Lastly I used the horse stall mats. I think I bought three. I cut them to width and centered them. This left me one foot short on each end so I used my third piece and cut two 1’ pieces for each side to fill in the gaps. I did this so I wouldn’t have any seams in the middle of my platform.
My platform is great but 8’ x 8’ is plenty big. The Cybex platform at the gym is 6’ x 8’. You could cut your platform down and then use one full length horse stall mat on each side.