Alumanim tears really easy, it was one of the first top sheets I tried when I was trying to save money. HDPE is soft and scratches easy and doesn't slide very good. You'll want to throw it away after a season or two. UHMW means Ultra High Molecular Weight Polyethylene, its like HDPE's big brother. It is more expensive but only by a few dollars a sq ft. and it's well worth it. It slides like ice, its impact resistant, doesnt scratch or gouge, and it lasts for years. It's been 3 seasons with heavy use and it's still in great shape.
But you dont want to waste your time with anything under 15 ft. 7 ft seems like a lot in your garage but when your sliding across it it's nothing.
ok, ill see if i can get my hands on some of that.
I would love to build a monster box, but i have no place to keep it besides the bed of my truck, I figure if it comes down to it ill build another box and just connect them with a slide joint. Im up at school and cant imagine moving a 15ft rain around, much less a 7ft rail.