Take a cheese like cheddar, one that melts well, and grate it or grind it to help it melt better, then stir it into the hot, drained macaroni with some hot milk. (You can have the pan over very low heat if you keep stirring.) That will melt the cheese and blend it nicely, and if it isn't perfect you can put the lot into a casserole and stick it into the oven for a bit. If you're doing the tuna bit, it will melt in the oven just fine.

The packages have a powdered cheese, which melts very fast indeed. It's bright orange and strange, but you may have something similar. The point is, it's melty...