I run 6 x GP2700 rechargeables, which easily do a whole weekend. Downside is, you need to remember to charge them - plus side is they outlast procells. But you can get procells off ebay cheap in bulk, and you know they're ready to go when you get them out of your kitbag!
I agree with Samwell though, the twin 9V's are capable of far less mAh than 6 AA's, so they won't last anywhere near as long.
In my opinion, change your board for the reloader sound activated one, I found that my batteries lasted alot longer when I did, maybe because it doesn't power the eye's that the Halo uses.
Hope this helps
Tim