Seeing cement blocks used for veneer pressing, I thought I would share a technique. When I'm up against a difficult oversized pressing, I use the ceiling joists in my shop. I make a caul to cover the layup, then cut 1x4s to wedge up against the ceiling joists. I cut them about 1/4" to 3/8" long, then bow them into place. If you don't have a big enough, or flat enough bench, I make an egg crate style assembly on the floor. I use 3/4" particle board and rip into 11" strips. I then cut vertical half laps in it and basically make a dry fit torsion box interior. This bridges across any out of flatness in the floor and I get a nice flat, no twist top.
Here is a pic of the first project I used this with. I ordered the diamond ribbon strip mahogany top, laid up on 3/4" mdf. Then built a torsion box interior, and 3/4"mdf bottom layer. The unusual aspect of this piece was the client wanted a minimum base under it, plus be able to have computers inside. I had a steel angle iron substructure built and anchored that to the concrete floor. We slid the base over it. Each end of the base has a door. Before the top glue up, I fixed steel plates with bolts welded to them between the torsion box webs, and screwed to the bottom skin. Drilling templates were made to transfer the holes into the angle iron. It was a big challenging project. We did several pieces for that office remodel in the hay day of my custom woodworking business.