Make your own: - 3/4 inch diameter PVC tubing with 1/8 inch thick walls. Usually comes in 100 foot coil for ~$12-$30 (find at local hardware store). To create a 60 inch diameter hoop, you'll want to cut a 188.5 inch length of tube. You can cut the tube with a PVC pipe cutter or hack saw. Slowly expand to full circle. You can use a heat gun or hair dryer to aid the process. Tape ends together. You can also connect the pipes with an inside PVC connector for less than $1. - 60x60 inch spandex fabric (as stretchy as you can find 15-20% spandex content). 60 inches is the typical width of a spandex bolt. Get a little extra just in case. Usually fabric stores have coupons for 40-50% off a single item ~$10-$20. - 12 large binder clips ~$5-$12, found at most stores. Clip spandex to PVC pipe, stretching to keep spandex tight. Readjust as necessary. Use chairs, stools, or tables to hold up completed gravity well. A more sturdy structure made of PVC pipe can be easily constructed. For central mass, a 2 pound weight toning ball works well ~$5, found at most big box stores. Marbles work well for test particles ~$5 at Toys-R-Us.