Hi, Nevin,
My suggestion is to get 3/8" - 1/2" plywood from Lowe's and have them rip 4 x 8 sheets with their panel saw. Secure your 1.5" foam to the plywood. The plywood will provide a screwing surface for anything you need to mount to it, and the foam should be deep enough for anything short of a chasm requiring a multi-panel trestle to span.
If the plywood gets uncooperative about remaining level, beat it into submission with battens or lengths of angle iron (which is what I did to a couple of wannabe roller coasters in my cookie-cut subgrade.)
Chuck (modeling Central Japan in September, 1964 - at the other end of Nevada)
I built the same kind of framework as you linked too (I actually used that same site as my "guide"). I found some 3/8" plywood at my local lowes that was pretty flat and not to expensive. The wood cutter was kind enough to cut a few sheets down to 15"x8' sheets that I used. It's been 4-5 months since I've mounted the plywood to the shelf and no problems with warping.
On top of my shelf mounts I first screwed a 2x2 wood stake (with the tip chopped off) I found at Ace Hardare, and then screwed the plywood to that. Then I glued a 1/2" thick of piece of wood (paint stirrers would work to, and free!) to the shelf standard and then glued the backdrop coil to that.
Also, to cut my trim coil I used a pair of scissors. Fiskars "Titanium Nitrade" to be exact. I found these at lowes and our basically our "household" set of scissors.
Shaun