I used 1/2" ply...or, 15/32" or something like that. Tried 3/8" (11/32", maybe?) and it was way too weak...like, popping and cracking under my feet. You wouldn't think there would be such a big difference in that extra 1/8", but there really is.
At some point I'm going to get a big sheet of scrap vinyl and throw it down in there (no glue), but I'm not in any big hurry for that.
At some point I'm going to get a big sheet of scrap vinyl and throw it down in there (no glue), but I'm not in any big hurry for that.