Good work!I often have a medkit in my backpack. It contains a space blanket. Probably, a bit better than a survival bag.
For sitting, I have a small mattress (something like 12x16 inches; 30x40 cm). It has a waterproof bottom so no problem when sitting on a wet stone. I cut it from a bigger sleeping mattress.
(those silver space blankets are fine, but they can rip easily. No harm in having both! The advantages of the orange one are: they are 2metres+ long, you can crawl inside for shelter and they are a big shape of bright orange to be seen)