We use the 'empty' milk cartons too. That said, some enterprising person could come up with a really great 'invention'.
I've thought of a tube made out of rubber or canvas, remember the old water bags we carried on the front of our cars for overheating crossing the desert?, that could be rolled up for easy storage and brought out and filled with water to weight down the rug. I'm thinking of something like those bright green hoses that sell for $19.95 only larger.....Dennis