Here's test fitting the plywood and hinge location.

Here are the 3/4" plywood panels, 16" deep each and about 47" wide. I've routed the edges except along the hinge side. The hinges are standard steel door butt hinges, $10 at the hardware store. I had to route a little channel for the hinge barrel as well as mortise the hinge plates.
Chisel for scale:

Test fitting underneath the folding platform:

Cut out the stock mounting point threaded plates:

Weld in some angle for a flat mounting point for the hinges:

Test fit base with folding platform extended:

Test fit folded:

More to come....