I decided to build the drop floor before I lower the frame onto the dollies for the rest of the project. I dropped the floor because I had to drop the roof by about six inches in order to build inside my garage. The drop floor area is between the bed and dinette. It turns out that my trailer frame cross members were in a perfect location that allowed me to bolt the sides of the floor box directly onto the frame. The trailer frame is 3" C channel so I filled the channel with a treated 2X4 cut to fit which would serve as the bearing surface for bolting the box side walls. I happened to find some 1X5 lumber which gave me the drop I needed and that kept the bottom of my box about 3/4" above the bottom of the axle which is the lowest point. The floor of the box is 3/4" plywood recycled from a shelf from my previous project. I had to cut it into two pieces and splice it in the middle. I probably overbuilt with 2" screws every two inches around the perimeter and six metal roofing angles bolted along the sides. I added an aluminum bar down the middle to reinforce the splice area. To install the box, I simply rolled it under the frame on a dolly and then lifted one end into place and used two trolley jacks to push the box up into place while I drilled and bolted the side walls.
I laid out the cut by drilling holes from below and measuring to the edges. Masking tape was the outer edge of the box and the cut line was 3/4" inside the tape.

I used a saber saw to make the cut (slow but effective). The two scrap boards were screwed to the cut piece to keep it from falling.

Cut complete. You can see the bottom flange of one of the C channels that will be filled with a treated 2X4 for bolting the box.

The box walls are cut and dry fitted to the hole.

The box is heavily reinforced. Wanted to make sure there was no give in this area.

Top view of completed box.

Dry fitting the completed box. At this point the box was very tight. Since I planned to paint the box and the 2X4's with several coats of Rustoleum, I took the 2X4 side rails and trimmed them down on my table saw by about 1/16" each.

The outside of the box and the two 2X4's were primered with Zinsser and given two coats of black Rustoleum.

I used two trolley jacks to push the box up into place. When the box was fully positioned, I drilled and installed three 5/16" carriage bolts on each side (I had already pre-drilled the metal C channel and 2X4 bolting rails.

Another view of the box in place. Note that I only bolted the two long sides of the box. I felt that would be plenty adequate.

The drop floor is installed.

Giving the floor the true load test. I will install metal edge trim around the top, paint the sidewalls, and install matching floor tiles later.
