Best suggestion is to use "tarp clips" about every 8 inches around the perimeter of the roof.
See https://www.amazon.com/Homax-5304-Locking-4-Pack-Carded/dp/B000PIGKSI/?tag=atomicindus08-20 for what I mean.
Then you save up milk jugs and use these partially filled with water. You can start with the jugs nearly empty, and then go around and top off the jugs a little at a time to increase the tension.
Once you have the roof placed and weighted, let it sit in the sun for a half a day before fastening it down.
Joe K


