Reply by pintodeluxe

The side sections can be held flat with figure 8 tabletop fasteners. The center section can be coaxed flat with a pair of cleats from the underside. You might lose a little depth in the center section’s storage area, but at least the panel would be flat. You will be surprised what a 1” thick cleat will do for you. If you hold them to the sides of the center panel, you would still have some good storage space underneath.

-- Willie, Washington "If You Choose Not To Decide, You Still Have Made a Choice" - Rush