Hardwood with an eyelet would work. Any thoughts on my original ideas to lightly screw in a metal plug with a hex hole,,,,as used on intakes to block unused holes. 5/8 and 3/4.

Doubt would make much difference if to block only one hose, nor which one. They are different sizes. Heater inlet/outlet. Likely could be a very loose fit,,,,as anything would block most coolant.