I have these shaft adapters that must have cost the taxpayer a few.
For shafts of 1/4on one end to 3/16th in. on the other the shafts are tightened with 2 allen head screws then the hole thing is ballanced by counter wights nice!
Bellow flexable center make it very smooth.
Measurering 3/4 by 3/4 in.
I have several available.
25.00$ each plus Shipping.