I initially cut the foam board with an X-acto knife and a 4' metal "yardstick" that kept me cutting fairly straight. Later, I got a quality box-cutter with a wider blade that was less likely to curve. I would do a small incision first, to get my path, then go back and cut the board more deeply with the first cut helping guide the second. It usually didn't cut fully through on the second cut, so I'd hold up one end of the board and let it sort of open up the gap and I'd run the blade through to cut that final thin layer of foam.
Despite my careful measuring, I seemed to have to trim most of the pieces once I got them into the trailer. Some areas had braces that required a little extra workaround, but in general, it came together fairly easily. I managed to only cut my fingers (thumb, actually) once but it was a pretty deep cut that took about ten days of Neosporin and band-aids to heal.
A can of Great Stuff insulating foam would fill the gap between the bottom of the foam board and the subfloor. I'd forgotten how much that stuff expands and what I left to dry had tripled in size by the time I returned a few hours later. That was fine, though. I took my blade and just trimmed off anything that came up above the subfloor level.
Installing the Reflectix roll insulation (it's basically bubblewrap inside of two space blankets) was fairly easy but really would have been better with two people. I could have used someone to hold it up as I applied the Reflectix foil tape, but I managed to do ok on my own. I checked with the company to see what they thought of my intended insulating plan and they said if I didn't have a good way to attach it beside the metal wall, this was the next best way. The lady who responded said my R-value in the walls would be 6.0. That's not bad. It will be the same in the ceiling, I suppose, since I'm doing the exact same thing. That's not great for a ceiling, but it's better than a tent's R-value. As this layer goes on, the inside of the trailer is starting to look like a lunar lander.
