I went to the local rubber shop, got 2 3' wide sections of rubber sheeting that has square grooves on one side. I got 2 12' long sections and 12 feet of 4" wide rubber strip to glue them together (its about 1/16 inch thick). I picked up some rubber glue from them but I think contact cement would work as well. I flipped the sheets over, put them together, sanded the area on each edge for gluing (and hte strip) I applied glue and let it tack up. I then weighted down the whole joint with boards and bricks and let it sit for 24 hrs.
I had pics of this but my e-cam batteries died.