Reddit reviews Southco Inc K3-1625-52 Rotary-Action Draw Latch 1.83 Closed Length, 450 Lbs. Load Capacity

The keyholes would be in the butt side of the joint. The edge of the boards that you want to join up together. You'd recess the keyhole into the wood and have the key at the right depth so that the edges join up nicely when the key(s) are in the keyhole(s).

As for something to attach to the underside, some kind of cam/lever based latch might work, something heavy duty like this. As long as you have some kind of solid system for alignment, that should work ok to hold it together I'd guess.