I have a cap over the back of my truck and spend many nights a year camping and sleeping in the back rather than setting up a tent. I really like sleeping in it, saves the tent setup effort and gives my truck an aspect of multi-purposeness and self-sufficiency. The cap is fiberglass and the bed has a plastic liner over it.
The only problem is that it's not at all thermally insulated (save the sleeping bag you're in) and my favorite camping is in the mid Appalachian mountain area where the temps at night go freezing as early as now and often get down even much lower. I have spent a night in around-zero weather (30ish give or take) in my 15 deg bag and a fleece union suit and I didn't enjoy it too much and I'm not even that cold sensitive.
So I'd like to go and camp a few more times this year until maybe early December. Is there a portable (so it's not permanent) solution to add some thermal insulation to the inside walls of my truck cap to make it sustain down to about 20 deg outside weather while keeping it around 40 inside. Since the space is not all that large (6' bed on a Tacoma) and the roof of the cap is regular, I imagine my body could warm up the area as long as this prospective insulating solution was in place.