I used a 30 mil cross linked poly, as used in toxic waste water ponds at shale drilling sites. Bought from Reef Industries in Houston, manufactured in Mexico across from San Benito, shipped to me in San Antonio. About a ton of weight, about 7,000 sq. feet, about $0.75 per square foot. It's a good thing my friend was there with a big loader/backhoe to receive it from the truck. This stuff is inert, nothing sticks to it good, but sticky rubber patches or sticky pvc tape covered over with more plastic and glue works pretty good. Sections are welded by electromagnetic heaters, you want that done at the factory. Onsite welding would cost thousands for crew, machine and travel. Another trick is to torch the plastic gently to create a glue-able surface.
