RE: flex cable question
For making a square end you need a fixture / a mold / and a xTon press.
I've never seen a commercial square-male adapter.
You cheapest option : find someone with a lathe , drill the square-propshaft to 1/4" ,
and glue the round end in with Loctite 648.
If the propshaft of the stingerdrive is a black-"chinese" eBay thing .. they have the 1/4" round version too.
You'll just have to order a new 1/4" round propshaft.