Quote: As previously stated, bondo is not for stuffing in holes. Here's what I'd suggest ....
With some sheet metal screws (wood screws will do in a pinch) secure some sheet metal behind the hole - as mentioned, a street sign does work well. With a large hammer, pound the repaired area in so that the heads of the screws are below the body line. Apply a sufficient amount of bondo to level the body panel. Finish as desired.
Remember, a paint job is only as good as the bodywork underneath.