I think any combo of those will work. I dont think Bondo will be an idea choice. I think lots depends on what you have available or what you feel most comfortable with. I am currently riveting the sheet metal on the outside. My sheet is short at the top so Ill be welding in some angle to get it somehting to rivet to at the top. Once the inside sheet metal is off I will start to pack it full of polyiso insulation and then put in the windows.
yes, i just removed the windows, cleaned the area with acetone, applied silicone caulk,drilled holes in the appropriate area and attached cut to size 18 ga steel squares to the frames using the window attaching screws