How durable do you need? Depends on the ground you cover. I'm often in spruce forest, sharp little needles and no way possible to clear them away. Needed poke-proof, wanted the heat boost. Regular cheap emergency blanket mylar sheets do not stand up to pokes and punctures at all.
Here's one solution from the hardware store: the heat-reflective underlay for floor installations. Your basic mylar sheet, bonded to 1/8" of closed cell foam. Foam side down is very tough, with the bonus of a little added padding under you. Cheap and ultralight. I've now made sitpads from this stuff too.
Another solution: something like the All Weather Emergency Blanket, a true grommeted tarp with a reflective side. You're unlikely to puncture this. Multipurpose advantages as a pitchable tarp and emergency bivvy. Great bit of kit for $15.