Here is an overview of how I hung two AI's over my 30"x30" cube. I've got access to some fabriction equipment at work that made this very easy to do, however you may be able to adopt some of the ideas to make it work with what you have available.
First, I made an "H" by welding pieces of steel strip measuring about 1.5" wide by 1/4" or 3/8" thick. Prior to joining, I milled slots into each piece that were wide enough to pass the AI mounting screws through. This gave me the ability to adjust the distance between the two AI units to get the best side to side coverage on my tank. I then used metal conduit (around 1.5" diam) from Home Depot as a pole/post to support the "H". The conduit slid over a piece of conduit of smaller diameter so that I could adjust the height of the lights over the tank. Finally, I cut holes in the large diameter conduit so that I could feed the AI power and control wires through the conduit. Guess I should also mention that I painted everything as well.
Hopefully the pics below help explain what I did and even if this doesn't work for you, maybe it gives you some ideas as to what will work.