hey Paul--
gluing pads together will work just fine, I'm told Gorilla Glue is good (available at Home Depot I know), and some others.

I think the 1/8" + 1/4" plan is sound, and for another bit of insurance hang a Neat Sheet or poncho liner or maybe even just a fleece blanket on the outside. I think you can manage this without sewing (but if you sew, then make channel ends to pull the thing together at the head and foot to close off air coming in. Otherwise try to effect this by tieing it.) Since you'll have the pad on the inside and your shoulders/head and feet will have plenty of insulation, what the outside layer can do is give you a bit of something extra in the middle.

You'll want your pad to be at least 25" across your shoulders, wider if you can, that's where you're likely widest and most likely to slip off the pad.

Something like this you'll be fine for sure. Probably you'll be fine without the outside bit, but when it comes to cold I'm a conservative sort, having grown up where there are two seasons : winter and 4th of July.

Grizz