I was the guy who wrote up the "AVOR" design as mentioned earlier - which has proven to do a decent job for me even if it looks like a disaster area.

However, one of the main things I would change if I were doing it again (and I don't have plans to do that anytime soon) is to go up from 3mm wall thickness steel box to 5mm. When cutting harder materials especially there is a certain amount of resonance/vibration that you can feel as a buzz through the whole machine. May not be needed for strenght but just simple mass. There's a reason why commercial milling machines are made of cast iron - yes, rigidity is one reason but simple vibration damping is another.

I can only think that 1.5mm is going to ring like a bell!