As a recent TT buyer I would prefer to have a nice looking, small patch that's watertight. One way to do that is to combine several of the suggestions.
1. Use a filled epoxy such as J-B Weld to fill in the damaged area. This will make it both mechanically secure and watertight.
2. Get a piece of aluminum flat stock from a hardware store, something in the 1/16 inch thickness range, and cut a square piece to fit over the damaged area.
3. Place sealant behind the patch piece and use aluminum pop rivets to secure it in place.
My reasoning is that unless you are willing to replace the entire thread plate area it will always look "patched"; you, as the owner, can be comfortable with a mechanically secure and watertight patch; and a future buyer will be reassured that the damage was addressed in a good manner.
Ross & Joan - 2014 185RB - 2012 Ford Escape V6/4WD