Step 2: This Crate
The two compartments are separated by a plywood "floor" that lifts up. The floor has circular thumb-holes drilled out (with smooth walls, no splinters) allowing a human to lift the floor anytime. The floor is supported on its edges by a molding-like feature of 1x lumber screwed onto the wall of the crate, just below.
It slots down over the vertical brace lumber, and is cut to have enough room to slide by even at a ~15° angle -- which is exactly the amount of tolerance required.
Shown here in the first image is the crate, before packing, and its future lid (to be screwed on) leaning on the side.