When I needed one there were a couple new ones on Ebay. I was going to make it from a bolt but found it first.
I've had good luck tapping on the top, then add tension to the bolt while tapping on the outside of the aluminum with a soft hammer. Not hard enough to dent it, just shock the metal. It almost always unscrews, you have to know when to stop and repeat. 6 point wrench for sure. And I like seafoam penetrating oil, I've had good results using it even after Kroil didn't work.