Sometimes, especially in urban settings, there is no provision for a campfire. Let me make a distinction between bonfires and campfires… a bonfire is very large and too hot to get close to, and a campfire is the right size to cook over. This reminds me of an old Indian saying: “the White Man builds a fire and backs away from it while the Indian builds a fire and hovers over it”. A campfire can be both aesthetic and practical.
The method that I use most often when I am an exhibitor is using a dedicated garbage can lid placed rim-side up on 3 rocks or bricks. This allows air-flow underneath it and protects the grass beneath it. The rim helps to contain the firewood. The lid alone will become scorched but to minimize that effect you can buffer the heat by lining it with small stones, dirt or sand before building the fire on top of it. Another convenient temporary fire tray is a metal wheelbarrow. It’s best to line it first with the materials just mentioned so as to protect the metal and wood under-structure. Terra cotta flower pot trays or chimnias should also be lined first because they are more fragile do to heat extremes.
In any case, these temporary fireplaces when elevated protect the ground or surface beneath while minimizing damage and can often virtually leave no trace once removed.