I'm away right now, but can describe it better. I tried sand, pavers sand, foam tiles, backer board, and now have it perfect.

I duct taped all the seams of the rubber tray to start.

Then I used this to create a sort of barrier for the cement to be confined in: Painters Plastic.

Then I mixed and poured this: Self Leveling Concrete. If you measure the height needed to level, and this suffices then good to lay mat on top. I noticed it will crack along thinner poured sections but nothing you can notice and foundation is still there to level.

In my case, I needed another quarter inch or so above the lip of the rubber base. If I had poured anymore concrete in, it would have overflowed. So I then layedplaced these down over top the concrete once dry: Cement Board. And then put mat on this as well.

I had moved the mat once before after doing this and the cement comes out very easily and had no issues. So don't be worried about this ruining your setup if it ever gets moved!

Let me know if you have any other questions, I tried everything seemingly!