We have done a couple repairs like this now for customers, we even had one repair that was 6' tall x 2' wide and this technique worked great.
Get your self some .060 aluminum, have it powder coated to match, use your discretion on patch size (don't make it a 2" x 2" patch) make it a decent size patch to it looks like it should be there. Then sand the area that needs to be patched, get some Pro form 2 part self mixing epoxy, clean the area very well and then sand the back of the patch panel. Make yourself some reference marks to make it straight, then put the epoxy on the patch panel and press in place. If the trailer is rivetless use some tape to hold the panel in place until the epoxy cures, other wise pre-drill evenly spaced holes in the patch panel and line it up over the damaged area and drill holes in your trailer before applying the epoxy. Then put your patch on and rivet it in place.
Hope that helps