Best material would be the drawn and heat treated stock the majors use for their wrenches. For a one off with the major diameter limitation you specify with only the end machined to a hex the quickest and most available stock to size is drill rod. A big plus is it can be heat treated, it's tool steel.
O1 or W1 would be good for the first 10-15 minute prototype, it's available off the shelf in the size you need for a dollar or two. Heat treat the hex end and give it a try. Or not, perhaps it would be good enough without heat treatment. The entire tool could be heat treated if required for higher torque strength. (It won't make it twist less.)
Torque requirement would be useful, that along with the diameter restriction would have added to the original request to get a good recommendation.