I use to use plastic,it is black on one side and white on the other side reflects real well.You can buy it in hydroponic stores where i live.It is about 3metres wide and what ever length you wont..
Nursery supliers would be able to tell you where to get it ..
Dont wory about heat i use to run a 1000watt mh in my 1.3m by 1.3m by about 7 foot tall set up wraped around alluminum square tubeing you just put it together like lego....very easy and cheap set up...