I would think the weight of the concrete and rebar should be great for
keeping the unit stable. I would think about putting a layer of something,
maybe a plastic under the plywood so you can move it later, hmm, I'm
thinking the plywood might not be a good idea at all, as it is against the
floor, which I assume gets mopped once in a while, and it is also against
the concrete, which absorbs moisture, so at some point the plywood will be
deteriorating and make the set-up unstable! I know, I worry a lot.