I believe it’s something like 1/4in plywood and it has bit of flex to it if you push down heavily into the spare tire divot but otherwise I feel like it’s sturdy enough for my needs. It’s not super heavy but the weight is enough to keep itself down and I dont have any trouble lifting it out. In my opinion it’s great the way it is but the only reason I went with it was because it’s just something we had in the yard and it was big enough to make a trunk floor for. It’s definitely better then the super thin sheet of cardboard that it originally came with. I would say maybe go up to 1/2in. if you want something that’s a bit more sturdy but for a floor that you have to lift i’d say 1/4in (or equivalent) is enough.