George,
It's been a really long time since I've messed with an Arc (or any light for that matter), but IIRC, the Arc has the same threads as the Surefire E series. That being said, you may be able to use one of the Aleph 1.5X123 tubes to get what you're looking for. If not, a tube could be made, but it isn't as easy as it sounds. I can see it taking a couple of hours of turning, if I started with a piece of solid round stock. As for the HA, getting a single piece anodized would be cost prohibitive. Besides, the anodizer that I was trying to work with down here got wiped out by Katrina. You might consider the use of another material. Maybe something like aluminum bronze. It has a nice color to it which you might find to complement that of the Arc, and the stuff is pretty tough too. If you wanted knurling, that would just add to the cost.
As for the total cost, I'd have to see. I did make a tube for an Arc 4 to accommodate a Pila cell a while back and I think I charged something in the neighborhood of $125 for it.
