you measured the flats, shouldn't you measure the outside of the threads?
I've used nylon spacers from the hardware store. Basically just a nylon cylinder on my hub, but that was for inside the fork (changing the dropout width). You could get a copper pipe and cut and hammer it to size, but I don't think it would take the twisting torque without some wear.
So, no - probably not possible. I can't think of anything that would take the torque. If it was steel, maybe you could weld some in there and drill it out, or just weld on a new dropout.
(I can't tell if that is a rear or front dropout you are using?)