I use one of two metal trays to confine coals.
Largest is what I call a "pig feeder". Actually a galvanized oil changing pan found at the local Fleet Farm. They work great, bulky though.
Recently switch to round stove burner covers found at Target. Four burner cover set for around $12. Light, easy to use, compact. A friend uses old road signs. Bends small sides to contain coals.
I have also utilized park BBQ stands. They are easier on the back because of the height. Grates require a metal container to confine goals.
Have also seen DO put inside small weber portable grills. Usually I try to utilize a fire pit to limit damaging ground cover.