I just went looking on other forums, was pretty sure I'd just replied to this topic. Yep, here it is, right below this one in a new thread.
I haven't had to do it, there's many options as long as they are thick and chunky to tack weld. A stock replacement type may not be the best depending on if there's rust or the nut seized or the spotwelds broke.
What I would do though, is probably use a hole saw rather than a angle grinder as often seen. Cut on a slight angle at the end so it cuts through on one side and leaves a thin flap attached so it can be folded back over and tack welded around to shut.
Or, if no access to a welder suitable for sheetmetal, have a look at body plugs like those glued into the floor pans with sealant on the production line, clean up the hole you have cut and drop a plug into it, looks stock, seals under carpet, can be removed and fixed if needed at a later date.
https://cdn11.bigcommerce.com/s-p5g7ozqt0j/images/stencil/1280x1280/products/10202/11600/CL2095C__32890.1658360795.jpg?c=2