You think ply is going to be dead flat? (uh uh).
I'm on table Mark 4, Mark one was countertop (curved like a skateboard course) and I have slowly improved each time.
For Mark 4 I have melamine board, 2 pieces 18 mm thick, glued and screwed from the underneath, screws every 12" in all directions to make 36 mm thick. Then 40 mm square cherry edging, glued and screwed in from the sides.
That is mounted onto the drawer base laying on an extra thick bead of silicone while its still wet.
Strategic weights around the top ensured it was level in both planes while the silicone dried.
I'm now working on the plan for the mark5. (lol)
OH, and I started out with the Kreg phenolic plate, but that warped. the free replacement from kreg also warped (not a lot, but enough to measure and i make a lot of small parts where dead level is important) so now I have an Incra alloy plate, which is spot on.