I often make a cheese sauce in the microwave. I never measure anything, I just put a big lump of butter into a dish with some milk and then heat it until the butter is just dissolving into the milk. Then I throw in a bit of flour and heat it some more, stirring occasionally. Once I've got a basic roux, I'll add some grated (or crumbled) cheese before a final short blast in the microwave and a bit more stirring.
I've not made macaroni cheese, as such, that way but I've frequently made a cheese sauce (often with a blue cheese) to throw over some cooked pasta and serve alongside a few rashers of bacon. Quick, simple and delicious!