i always use an offcut of builders black plastic, cut it to any size you want and it rolls up to the size of a bike pump. have a word at your local building site and look in their skip
Get yourself down to Wickes and buy a pack of their secondary glazing film, also known as polycryo in the lightweight backpacking scene.
I cut mine down to roughly 1.5m x 2m, added a piece of gaffa tape on the corners and poked a hot peg through, total weight all in 44g
Thank you everyone! I will probably go with this idea Shewie- thank you!! Could this stuff be stuffed into something or would it have to be folded down? Not a huge problem either way..
Either or really mate, I fold mine first until it's paperback book size and then roll it up, that gives you something about the size of a Smarties tube. You could stuff it into a small sack which would give you something the size of an orange.
Obviously when you're sleeping on it the underside gets wet from condensation, either peg it out the other way up in the morning or try and fold the wet faces into each other so you always have one dry side, probably more important on multiday trips than just a weekender though.