I don't know about quantities, but I'd make a small batch of white sauce (roux & then milk, cooked until thickened), then add some shredded colby or cheddar & then some cream cheese. I do that for my mac & cheese. I find the cheese melts smoother if I toss the shreds in a tiny amount of flour before adding it--thickens a bit more, too. Then add to the other ingredients until it looks right. Good luck!
As the arc of history bends towards justice, it's a new, more progressive day. --Steve Benen, The Maddow Blog, 11-07-12