I have a Cumulus 350 (older version). I purchsed some 1 inch wide elastic which has button-holes every few inches (see attached image). I created 3 x elastic loops which go around my sleep mat. I then attached some mini-toggles to the quilt, 3 each side, using the fabric tabs already provided by Cumulus. Once my mat is inflated and the elastic loops are fixed, I button one side of the quilt into the elastic loops which keeps it fixed to the sleep mat. If its cold I can button up the other side and if it gets really cold I can button both sides underneath me like a sleeping bag.
I can't remember ever using the top buckle.
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