Drill a hole in the center and put a bolt in it. If the cap is thick enough you could thread the hole, or else just use a nut. Locktight and some rtv silicone to finish things off.
Another possibility would be to epoxy the key onto the cap.
Welding a piece of aluminum hex bar would do what you're looking to do, but the cap maybe too small and thin for welding. Doing it would probably require someone really good at welding.
OR (just thought of this), get some hex bar (
www.onlinemetals.com, is a good place), drill the cap and the bar, thread the bar, screw them together from the inside, use epoxy instead of locktight on the threads.
There are also some solders for using on aluminum, but in general solder and aluminum don't get along with each other well.