Looks like a 4' X 20" blanket
goes for as low as $350. If you want the type that does not require a mask and gloves for handling.
Or $180 for a 2'X3' pad of this
To cover shoulders to upper thighs, I'd probably prefer 40", and another few inches of width would be nice as well, but it is $30/ sq ft.
Making a vest with a long tail would probably be a lot cheaper if you stay on your back when sleeping.
It looks like 5mm of that stuff will insulate ridiculously well.