1. Get a small length of one inch sch 40 PVC pipe.
2. Cut pipe to length to fit the depth of the counterweight (cw).
3. Cut a slot about 1/4 to 3/8 inch wide the length of the PVC pipe.
4. While squeezing the pipe, slide the pipe snugly into the cw hole with the slot opposite the securing bolt.
This assembly should slide easily onto the Atlas shaft, and, when the cw fixing bolt is tightened, the PVC should tightly grip the Atlas shaft, with the slot closing somewhat.
It worked well for me, to have something lighter (7 lbs) than the 11 pound cw. YMMV.