I don't remember what it was called, but I found something in the roofing section at Lowes that worked. They had a few different shapes, but basically it's long plastic almost half tube like thingys and they cut fairly easily with scissors (their like 10 feet long at the store). I didn't cut my canvas quite the right size though, so because of that they slip and I have to put suction cups on either side, but it works.