try google for cage nuts
fits a square hole
https://www.belmetric.com/index.php?...orgfd9uoq6ql33
clip on
https://www.bhphotovideo.com/c/produ...2_clip_on.html
weld on from back side
https://www.opgi.com/chevelle/C241477/
another style of clip on
https://www.auveco.com/cage-nuts
another style of weld on
https://www.classicfasteners.com.au/cage-nut/
I was thinking the same as Clarence with the steel plate behind. nice and strong. I have also used a nut welded onto a short piece of flat bar so I can get the nut into position by holding the flat bar. once the threads are started the flat bar turns and jams against a closeby object and acts as a "wrench" to hold the nut until tight. some cars use this style of idea for holding the bumper bracket nuts on inside the frame. you may be able to make some up quick and use a body washer as a back up so the nut doesn't pull through the steel bodywork when bearing down against. glue the washer on the back side of the hole first so it doesn't fall down. I have also used a piece of a nitrile glove inside the box end wrench to hold the nut from falling out until the nut is in position for the bolt. a flange nut is handy here so the nut doesn't push right through the box end.
https://www.grainger.com/product/FAB...-Flange-41MG56