A friend of mine has made a bunch of these for her family. If you take good care of them they should last a dozen trips or more. I used 2 "Heatsheets" emergency blankets and taped them together with "Frost King" waterproof and freeze-proof tape. The sheets are about 4 bucks each and the tape is about $3 plus. I also strengthened the edges with some regular packing tape. Obviously it won't take a grommet or any kind of sewing so you need to tie a couple of knots in the corners then tie your guy lines over them.

Here's the blanket: in the USA you can get these at Dicks or other camping stores:
http://www.hammockforums.net/gallery...hp?i=15083&c=7

Getting it to lay flat is important. Carpeted floor won't work. My friend tapers hers to the wall with low-tack tape. Luckily my work has a nice big cutting mat:
http://www.hammockforums.net/gallery...php?i=15085&c=

The tape is 2" (5cm) wide. I was trying for an inch a side and I just laid the other sheet down edge to edge. Frost King tape is actually pretty low tack when you apply it, so you can carefully peel it back and re-set it if you don't gt it right. I assume it cures over time.
http://www.hammockforums.net/gallery...php?i=15086&c=

I flipped it over and taped the edges on the other, then tied knots in the corners. I will hitch some shock cord over the knots.

http://www.hammockforums.net/gallery...php?i=15087&c=

The last thing I did was to apply some clear packing tape along the edges of the silver side. It might have better to do this before the knots, but we'll see.

The tarp is hung on a diagonal. It measures 84" x 112", so it's plenty big. My friend cuts hers down first so the final dimensions are 84" x 84", but I couldn't be bothered. She usually has the shiny side down, which apparently helps reflect some heat back as well.

All for 10 bucks approximately!