This was a staple in our house growing up, usually a once or twice a month treat on Saturday nights (with fried chicken and tossed salad), and it definitely found its way to our table every holiday. I don't usually measure the ingredients when I make this, so you may want to adjust this to taste.
1 lb. box elbow macaroni
8 oz. Velveeta
4 oz. cheddar cheese, cubed or shredded
2 oz. pecorino romano
1/2 c. margarine or butter
1 c. milk
Salt, pepper, garlic powder, and onion powder to taste
Cook macaroni to a bit less than al dente (you don't want it to overcook in the oven). Drain and return to pot. Add butter, milk, and cheeses, and cook over low heat until combined. Add seasonings to taste. Transfer to an oven-safe dish coated with cooking spray and bake, covered, at 350 for 30-45 minutes.