My worktable uses a 2 x 4 frame with 3/4" ply, good both sides (that's what they called it at the lumberyard). The frame was built as straight as possible. Then the 3/4" ply top was levelled by shimming underneath as needed. It's been dead-level and flat for 15 years...and I check it at least once per year...I've used it for models and my RV-9 project. The big 'uns need a flat worksurface, too.