Stacked Pads
Here's a suggestion I saw somewhere on the web. I wish I could find the site so I could simply direct you there, but here goes. The suggestion came from a hiker who had discomfort in his hips and shoulders while sleeping. He experimented and concluded that the discomfort was from lying on a flat pad that did not support the arch in his back, and hence, all of his weight was concentrated on his shoulders and hips. His solution was to use stacked pads.
Start with a 60-72" long pad (Ridgerest, ZRest, Mt. Washington, etc.). Cut the pad in half, and then cut one of those pieces in half again. You should now have one 1/2 and two 1/4s. Place the 1/2 down first, then place the two 1/4s down on top of the 1/2 so that the 1/4s overlap each other. The result is something that looks like this:
____________zzzzzzzz_____________
_____oooooooooooooozzzzzzzzz_____
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Perhaps not the best illustration, but here is the key:
x = 1/2 pad
o = 1/4 pad
z = other 1/4 pad
_ = nothing (ignore this, it is only there to take up space so the illustration comes out correctly)
The result is a nicely shaped curve that supports the small of the back very nicely. The result is that your bodyweight is dispersed over your entire torso and hips. This works very well if you sleep on your back or on your side. It does not help much if you sleep on your stomach. There is no need to tape the pads together. Friction alone will keep them in place. You can customize the system by choosing the pad length and thickness, as well as how you place the pads to conform to your own back. I've tried this at home, and it is surprisingly comfortable. I have yet to use it in the field.
If this proves to be as comfortable as my initial testing leads me to believe, I plan to abandon my Thermarest for good because this system has a lot of benefits:
1) Cheap
2) Virtually indestructable
3) Light
4) Customizable
I wouldn't worry too much that the final length of your sleeping system will be 36" or smaller. Unless you are very tall, the length from your neck to your hips is probably not more than 36". You can use your empty back for padding under your legs and feet.
John