Best thing I can suggest is to use closed ring crimp connectors on the inner and outer conductors of the coax, and then use the appropriate size screws (bolts) and nuts to attach them to the aluminum. Depending on what material the crimps are made of, you may experience some electrolysis or galvanic action, best minimized by using a paste that is made to go on all the connection points. It is readily available on the web or in many hardware stores. Next time you are in a place that sells TV antennas, or a ham store, look at how they are done. Most use rivets, which is also doable, but for the DIYer, I think screws will provide a better long term connection.

But then, I could be full of shit, too.