It should be a 5/16".
You can take a standard L shaped wrench and turn it into a nice T handle wrench. Take a 5" piece of 1/2" PVC pipe and drill a hole in the middle of the pipe on one side. Epoxy the short side of the Allen wrench into the pipe using epoxy putty. Loctite makes a very easy to use epoxy putty that you just cut off a chunk, kneed it together, and mush it into place. Available at Walmart, Lowes, and others.
good luck-- Mick