I built my table with some low density fiberboard with melamine coating on both side. Pretty flat, but to keep it that way, I cut some 1x3 into strips, flattened one edge on the jointer, then drilled holes vertically part way through each board using a drill stop to avoid drilling too deep. Positioned and clamped the boards to the bottom, then drilled through the remaining thickness, about half an inch into the melamine. Screwed the thing together without glue and it's remained flat for many years now.
My table folds down, but since I almost never fold it down, a number of items are stowed underneath it, including the compressor.
The more I do, the less I accomplish.