Next step was to get some lumber, resin epoxy, roofing tar, nuts, washers and bolts. I used 1/2 thick first grade plywood and 1"X1" pine to frame it.
The plywood was cut, glued and screwed into the framing. Notice the notch made to clear the wheel. Once the lumber was all put together, the top of the floor was epoxied using fiberglass resin to seal it.
The next day we flipped the floor and rested it on the frame to apply roofing tar to the bottom.
We used 3/4 of a gallon of the stuff. Be generous, you want to seal that underbody real good. Remember that water will be sprayed up there at highway speed. I'm not done yet for protecting from water spray in the wheel wells, I have another trick up my sleeve, but that will be for another post.
The floor was then bolted to the frame and it now sits ready to accept the walls. Many thanks to the good folks at Remorque BGS, they are very good people and they do cater to the hobbyist as well as the pros. Go see them.