You can fold the whole ridgerest into the back of the pack, but it will probably fit better if you cut some off (the foot section of the pad is not really necessary).
The z-lite would be fine too, and I don't think the r-value will be noticeable, especially with the air trapped by the convoluted foam adding a little heat and much more comfort. You could also consider the Gossamer Gear Night Light pads (my personal preference).
The Cordura might be a little tougher, but in all likelihood, you're paying for the brand name.
My question for anyone who's made one:
How easy would it be to scale the pattern down and make a ~2500cuin pack? I've never made a pack, so I don't know if it's the simple matter of making the sides a little smaller in each direction, or if there's more to it than that.