There is always a solution... there is always a solution... there is always a solution...

Returned from the festival a few days ago, pleased to discover that after ten days of rain and wind the device was still structurally sound and hadn't exploded even once.
However, as I was stripping the reflector to upgrade a bit of wood I made the potentially catastrophic discovery that mylar sucks.
Every single piece of dust, every spot where the metal mesh touched it, every spattering of guano had somehow eaten a tiny hole in the reflective surface, leaving only the clear plastic backing. Holding it up to the light the damnable thing resembles a starry night sky of disappointment.

The two days of internet research which followed would be best described as 'feverish'; due partly to the fact that my stress levels always elevate a bit towards the end of projects, but mainly because I knew full well that:
Mylar, my default option from day one, was now going to be completely unusable,
The list of alternatives was very small, possibly zero,
And the inability to locate one would pretty much kill this entire project.

As of this morning the list runs to just one item: aluminium kitchen foil.
I've always heard that this is about the worst thing you can use; it's a bastard to work with, scatters the light too much, and is poorly reflective. Early research on marijuana cultivation forums seemed to confirm it.
However, closer reading turned up that there's no real reason this should be the case, as aluminium is one of the most reflective metals, running about 85-95% reflective through white light and well into near infra-red.

Caught a train to town, got a roll of the most reflective looking stuff I could find in the supermarket, taped three lengths together and whacked it in the assembly.
The results are very interesting.

The focus is about half as tight as the mylar thermal blanket, ie 10-12 mm as opposed to 5ish. Still pretty good. However the amount of energy is (according to my incredibly precise method of sticking my hand in and seeing how much it hurts) a lot higher. Like, 50% higher, even at the half concentration.

This, I think, means two things.
The thermal blanket, as well as sucking at durability, also sucks at reflectance.
and
Aluminium foil is a lot more reflective than it's generally given credit for.

I'm really happy to have discovered this, as though it requirs a somewhat careful workflow to avoid tearing, foil is by far the most accessible and ubiquitous reflector material in the world. That it turns out to also be one of the most effective has a lot of ramifications for home made solar.
And, yes, I'm aware that I hardly discovered it. About 90% of projects I've seen have used kitchen foil, but pretty much all of them have made what looks to be the mistake of gluing the foil to a backing material. It desperately wants to crinkle, and it's the crinkling which makes it almost useless. -You've got to stretch it-.
It also loves to tear, but taping the edges and any holes seems (touching wood) to resolve this. If it doesn't hold up in the field I'll just cover the entire back with stickytape. It's cheap enough.

So, if Google led you here because you're researching reflector materials, know this: aluminium foil rocks. Use it.
(There's pretty much no other affordable option.)