I think your best shot at cutting that thick slab of aluminium is by using a (premium) jigsaw.
You definitely need to be able to control the speed and the amount it 'eats' with every stroke. Normally the blade will go back and forth about 1 cm (as well as up and down of course) which should be tuned back to almost 0. This is called the action of the blade. Cheaper jigsaws don't have any action which sadly enough results in a lot faster wearing blade, so don't set it to exactly 0, it really needs to move just a bit back and forth to keep pressure off the teeth when pushing the blade down.
The speed should be as low as possible, otherwise your blade will burn instantly and won't be able to cut more than 3 cm of aluminum.
In general just let the saw do the work, if you start pushing (because you're in a hurry) you'll burn up the blade as well.
 don't forget to buy a special 'metal' blade, the finer the teeth the better.
By the way, do you have any plans of isolating the whole box as well? I recon with the aluminum being -18 you'll loose a lot of energy because it's effectively cooling your room Edited by Jetskyer - 7/20/13 at 10:11am