We insulated and topped with 1/4" plywood, and now: we have a floor!
For the insulation step, I used some
epoxy from Home Depot to stick the foam to the floor--it came in 8-oz kits, and I bought 2. Three or 4 would actually have been a better choice, but it isn't like the insulation panels are going anywhere, so I'm not going to lose sleep over it.
The foam panels are slightly thinner than the 1 x 4s, so I had the bright idea of using
Puff-e-foam to fill in that shallow cavity above the insulation. I was thinking the challenge would be to weigh down the 1/4" plywood so that it didn't bow up as the Puff-e-foam expanded. I had
WAY overestimated the power of the Puff-e-foam. It turns out, 1 can contains a pretty minuscule amount of foam--not nearly enough to form a solid layer across the surface of 1/3 of the trailer. So we used it around all the edges and filled in the center with whatever was left. Flexing upwards from the inexorable force of expanding foam was SO not an issue.
I had only bought 1 can (again, totally overestimating the awesomeness of the Puff-e-foam), but fortunately, Mr P had a couple of spare unopened cans kicking around the garage. We used all 3 completely, and still didn't achieve a solid foam layer.
But we topped with the plywood and weighted it down, so hopefully the foam dispersed horizontally under the weight to provide a little extra support for the floor, which was my main goal.
We fastened with
PL Premium (I'd bought 2 tubes--also not really enough--there is a theme here) and reinforced with a
brad gun.
The workstation--note the plastic swathing--I was just sure that foam would be oozing and squirting and exploding all over the place, which is also why we worked outside. Eh, well.

The finished product, curing. Next step will be to coat with The Mix and screw to the trailer.
And then--holy cow!--I can put on some walls!!

PS--You'll note in the foreground a gallon jug of Titebond: my first full gallon of TB3! It feels like some sort of milestone. No more weenie little tubes for me--I'm swimming in the deep end now. I assume they'll be sending me some sort of certificate or diploma in the mail? And are there some initials I can now use after my name? "Plectrudis Jones, MD, PhD, OBE, MP, TB3..."