Starter Relay wire question.

My recollection is the connector at the stud was originally a ring terminal. Advantage of a ring terminal is more surface in contact and a very positive connection.
A good place to buy a ring terminal, but connector, and a foot or two of 10 gage wire is a marine/boating supply store. Marine wire is slightly heavier than SAE and has tin coating so less likely to corrode. Also, marine wire has insulation suitable for oil/fuel environments as well as the heat of engine compartment.