In a medium bowl, toss together frozen french fries and melted butter.
Divide the french fries between four 12-inch square sheets of aluminum foil.
Fold the aluminum foil up and around the french fries to create a packet or boat - leaving just a small bit open at the top to allow steam to escape.
Place foil packets over indirect heat on a grill or campfire grate and cook for about 20 minutes, until fries are crispy and beginning to brown (you can cook longer or to desired crispness.
Open each foil packet and cover french fries with two slices of American cheese.
Close the packets back up and return to the grill (indirect heat) until the cheese is melted.