BTL (Bend Tarp Liner) makes lightweight liners that are guaranteed for 10 to (I believe) 25 years UV rating. They last a LOT longer than that if you cover them with soil. They also don't require a goetextile fabric underlayment. The thinnest one is very lightweight, I had them spec out one for a 1.25 acre pond and it weighed around 4,000 pounds - all one piece.
