I did it (not on my jeep) by c-clamping the metal tightly, tack welding every 3/4" or so, then peening the sheetmetal down in between the spotwelds until it was fully in contact with the box tube for like 1/4" back from the weld. Then aimed the welder as far away from the sheetmetal as I could (so like the opposite angle from where you'd normally point for a lap joint) and moved fast/tack-tack-tacked it to let it cool down.
It still sucked.