It's a pretty simple recipe: 1 part uncooked elbow macaroni, 1 part (or more) cheese, 2 parts milk. Melt a couple or three tablespoons butter and use to grease a disposable aluminum pan of a size sufficient to hold the macaroni, cheese and milk.
Add the macaroni, add the cheese, pour milk over the top. Salt and pepper.
Put it on the grill at 300 to 350 degrees and cook until the macaroni is soft about an hour or maybe a little longer. The pictures is a fairly shallow cake pan so it cooked pretty fast compared to my previous time in a deeper pan.
Last night I used about half pre-shredded sharp cheddar, half pre-shredded colby/jack and then added some chunks of random cheese the lovely wife had cut up in a container in the fridge - I don't even know what kinds of cheese they were.