You could use a small triangular file to shape the existing hole into a female hex and use a hex rivnut.
Flat Head HEXNUT Body is provided in both a standard and heavy hex design. The heavy hex design features a larger body diameter that results in increased thread strength and torque capability. The hex body design provides the ultimate in spin-out resistance when used in a hex hole. The RIVNUT® is designed for use with Property class 5 mating hardware. A grip ID mark is included on the head of the fastener for grip range identification.
source: RIVNUT® Flat Head Hex Body
Rivet Nuts (aka RivNuts) are also available with a spline that holds the RivNut instead of a hex. If you go this route, you might have to drill the hole to the correct size.
Some RivNuts need an installation tool. Essentially, you tighten in a bolt that expands the bottom of the RivNut so it cannot escape the sheet metal hole. See the difference between the second and fourth image above.
I got familiar with this fastener when I installed some auxiliary bash/skid plates under a Ram 1500 4×4 so as a fastener they can take a fair amount of stress.