I think most people (including myself) use mylar. It comes in rolls that can be cut to any size, and is cheaper per square foot than car sunshades. I got mine from a local hydroponics shop, but you can buy it online from any number of places.

I watch for the emergency thermal blankets to go on clearance. They are strong mylar and very cheap.

Way cheaper than buying mylar at the hydro store. I think I may have enough to cover a warehouse, LOL.

Here is a link that might be useful: My Hydroponic Adventure

That most likely is mylar covered cardboard. Mylar can be found fairly cheap at hobby and party stores. It's the same stuff fancy balloons are made from.

I DO use auto shades.... but I got them in the fall on clearance for like a dollar each..

I use car sunshade to reflect light to making my fluorescent light setup more effective.
but my car sunshade material is silver nylon instead of the bubble car sunshade.
i think the link you show is bubble sunshade,
both the silver nylon sunshade and bubble sunshade can reflect light, but silver nylon car sunshade is more durable.
â¢This"Satin silver collapsible fabric auto sunshade"made of 190T Polyester with Silver reflective Coating to relfect light.
â¢silver reflective surface and black backside for virtually 100% sun reflect.

Here is a link that might be useful: Here is a link of silver nylon car sunshade