gro stores sell reflectorized mylar thats pretty thin and light, in 4 foot wide rolls. Theres some mylar duct tape thats perfect for the seams. This mylar is a pretty wide spectrum reflector and stays cool.

Aluminum foil, for example is actually a color and absorbs the rest of the spectrum, and gets hot shocked. Flat white is generally a better reflector than mirrors, but shiny white isn't as good for some reason - I think whatever makes it shiny absorbs some light.

a rock behind you is a good reflector but of course you can't take it with you, so you have to decide just how complete of a shelter do you want? a 4x6 foot piece of mylar might be fine with grommets in the corners.

Jim grin
_________________________
These are my own opinions based on wisdom earned through many wrong decisions. Your mileage may vary.