Ok, here's what I did with the space bags.
i bought the under bed storage bags (sold on QVC) since they are in fabric "boxes".
In one bag I put batting like the warm and natural that is NOT in plastic manufacturer bags. It sucked down and I zipped the outer bag shut. I noticed after a day or two the contents had increased in size but was not causing the fabric top to begin bulging.
In the other bag I put batting that comes rolled up in a plastic manufacturers bag. I did open the individual bags but left the batting inside. It did not suck down as tightly, nor did it stay down as well. The zipped shut fabric bag began to bulge by the following day. I think the fabric outer bag has made a little difference in how much the bag will expand. I will keep them, because they stack under my long arm table so they are out of the way.
I have a huge roll of warm and natural. My husband got a long piece of 4" PVC pipe and 2 car jacks and made a stand for the roll. He put the pipe through the center of the roll of batting and then set it under my quilting table on the two car jacks. That works ok. It isn't as secure as the fancy roll holders you buy with the longarm machine, but it works just as well.