In Lowes or HD in the tool section there will be a small display or product made by "General Tools" and they make a grommet tool http://www.generaltools.com/directory.asp?action=ppage&pnum=58&sectionid=4
You could use small pvc pipe since it will bend to make hoops if needed. I was thinking of making large hoops that fit on the bed of my truck like a covered wagon and making up a tarp to cover it.