In addition to DonC's suggestion for some new, non-FWC ideas, I thought throughout the day, while driving up through Wyoming, about some possibilities.
1. You could use the 1.5" solid foam wherever it fits, and then go with 1" for the edges, filling in any gaps on the upper roof side, with fiberglass insulation, going from 1" back up to 1.5".
2. Same as above, but get 1/2" foam board for the edge sections, stacking it by 3 X, and doing a step down approach as you get to the 1" edges.
3. I think this last one might work best. Cut the 1.5" thick foam board to size, to fit in all the outside transition sections first. Then take those edge sections, and stand them up vertically, mark 1" on the edge needing to slope down, and approximately where the slope starts mid-board, so it would give you some guide marks to represent a wedge. Then use a good ole carpenters wood saw to take the slice out you need to remove. The foam board should cut easily, and you should be able to make more than one pass until you get the slope down to where you need it, probably roof side, while maintaining the flat, even side for the inside ceiling.
I would also try hard to make each section of foam board throughout, as snug as possible, so they stay in place, by tension.
This should also keep costs down, since you would only have to buy the one thickness, 1.5".
Hope that all makes sense, without a picture. Feel free to PM me if you want to discuss further.
Oh, and to answer your other question on attaching the wood slats on the ceiling, I just went with the 18 gauge staple gun. After being in cold weather for my first trip, I could imagine seeing screws in the slats potentially forming condensation, or perhaps ice during the night time, as there's much more surface exposed than staples. That aluminum tubing transfers cold temperature quite efficiently, so anything touching it will become close to the same temperature, and bringing that cold to the exposed surface of the screw head.
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Edited by PokyBro, 08 November 2016 - 04:31 AM.