First up is the Red Spot. With this one you need to drive your car onto the stringer base. Not that convenient.
Next is from Bear Archery. No car needed. You use your own weight to bend the bow.
Last one is homemade by Roy Hoff, editor of Archery magazine. He based his design on the Red Spot stringer. Could be a problem if there are no trees around. Phone pole or lamp post maybe? He did state not to stand directly in line with the bow. He said the woven strap could be worn as a belt. At least you know where it is. From Archery, February 1960.