WELDING TIP -
A time proven trick when welding thin sheet metal is to back up the area with a sufficiently sized piece of copper.
this can be something as simple as a piece of flattened copper pipe or a fitting.
devise a method to hold it behind your welding area, perhaps attach a long rod handle, hold it in place with magnets, tack weld a thin piece of steel on the backside to hold the copper temporarily in place.
A quick Google or YouTube search will find more information.
Larry