you can use Styrofoam or black rubber door strips. It helps fill in any pressure points.
You don't need to support the entire floor of the tank.
The edges and some cross beams. Using sheet plywood with styrofoam will ensure even pressure across the floor of the tank.
If the plastic is thick 3/4 or 1 inch its not going to give much.
My stand is a 700 ilb powder coated steal. Its open underneath with two cross beams. If I did not have sand I could see all my tank from below.
No foam used either. But its dead on level and 1.25 inch think on floor sides and top.
I had a glass tank leak because I did not use a foam or filler.