I'd steer clear of build a CO2 laser unless you really know what you're doing.
EVEN if you did have it fully working, and a suitable cooling system sorted; beam delivery would be a massive headache.

Fibre coupled lasers, or even just fibre lasers, would be a better choice.

Those hacked DVD diode lasers are no good for aluminium as stated.
Don't go on eBay and find some 20-40W diode bars either, they require a significant amount of carefully placed optics to get a useable beam out of them. So don't buy; save your money!

I'm currently building a ~25W, 532nm (green) laser... (many kW peak power) for outdoor displays; it's around the size of a small fridge/freezer currently. Nearly there... :whistling:

Best Regards,
Dan

http://www.eonlasers.co.uk