Focuser hole in aluminum tube - How?

What is the 'best' way to make the focuser hole in a 12" dia. .094 Hastings Aluminum tube?
My last attempt, in a 6" dia. .094, used a round metal hole saw and was messy and needed a lot of filing and cleanup. The center bit drill chewed a bigger hole in the aluminum and the hole saw chattered around a bit. Fortunately, I used one that was smaller than needed so filing cleanup took away most of the damage!
I expect that a 'Greenlee' style threaded hole punch would distort the tube.
I'm putting in a 2" DAR dual Helical Focuser, so the hole will probably need to be over 2 1/2" (I'll get an exact measurement when I take it off the old tube!).
I suspect that there's no easy way, but any helpful hints would be appreciated.
Thanks.
Mike
6" f6 homemade on old Meade GEM
10" f4.5 homemade on old Meade GEM (WSP2008 Version!)

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I agree with the previous posters, you have the right equipment for the job already, you have to go very slow with the drill speed and use very light pressure. Avoid the temptation to rotate the bit around and lube the blade with 3-in-1 oil or mineral oil. The slow and light pressure is the key, patience, Grasshopper!

I've had success by pouring a disk of plaster inside the tube where the saw will penetrate (same idea as a block of wood but a much closer fit). Clamp the plater block to the inside of the tube while drilling. This gives the pilot bit and the "uphill" sides of the hole saw something to guide in after they penetrate; prevents the chattering. And the other advice is good: go slow, light pressure, lubricant. Hope this helps,

- Tim Iafolla