5/16" hex rod will do it.
Or a 3/8" diameter peice of tubing or rod with a 5/16" hex ground on one end. It don't have to be a perfect hex, it isn't hard to do on a bench grinder. A round rod isn't gonna hurt the bushing in the block.

I made the one in this picture with yellow tape about 15 years ago and have used it to prime hemis with 60 weight oil in them dozens of times. The yellow tape only idenifies it as a tool, it's not on there to protect the bushing.

Here it is stuck in a 440.