I upgraded my Coghlan's orange survival-bag by inserting a Coghlan's space-bag in it and taping them together with duct-tape on the inside.
When repacked, It's about the same size as the original packaging of the survival-bag. Just thought I would pass it along.....

BTW: It would be grate if AMK would also produce a Heatsheet-bag.

If you want a spaceblanket survival bag, they are availible off the shelve. They are smaller and lighter than the Coughlans survival bag. If you want a serious survival bag, which is beter in protecting you, you might consider the blizard bag. (beversport has them)

Mylar blankets are not very good at reflecting body heat. The most effective part of the mylar blankets are it's wind and waterproofness. Which a normal plastic survival bag already does. So not much to gain there. You could leave on side off you mylar/plastic side open, so you can fill it with insulation, but 2 light survival bags put in to each other, would perform much beter.
Having the mylar blanket seperate from the bag, would keep more options open. Like using the mylar blanket to cover a lean-to before you toss debris on it. This way you will have a good heat reflection if you have a fire going on and makes waterproofing a lean-to much easier.