Unless I've missed the point, those green wire grounding pigtails you can buy at the hardware store are made of #12 stranded wire. One end has a spade lug at one end, and an eyelet with a green screw at the other.
With older boxes, I've put the screw into the otherwise unused threaded hole meant for a cable clamp. The box has a clamp on the other side for the cables. I think that this is okay, yes?
