XS2 tank. Hard to tell from this pic, but the filler neck is very noticeably caved in. (Note the welded seam lines indicated by the blue arrows -- more on that in a bit.)
The next three pics are ganged up for comparison, showing before, during, and after my dent repair.
The repair began with this piece of scrap metal I found in my Big Box O' Bits and Pieces which is perfectly sized for inserting into the neck opening. All I had to do is drill a hole in the middle.
The hole was drilled and then angled so that I could put it on my slide hammer and then get it into the tank. Just a few whacks got me to the middle pic in the comparison sequence above. (The slide hammer hook tool on the left was used for a little fine-tuning later on.)
The problem at this point was that the welded seam area did not want to pop up.
So, I did some spot-prying and spot-squeezing along the seam with channel locks.
The channel locks had a nut taped to the end of the inside jaw to get past the extended inside lip of the neck. And the piece of scrap metal had a bit of inner-tube taped to it to protect the top of the neck.
Didn't take long at all, and the final result just needs a little light glaze coat of filler to be perfect. Very satisfying for a first attempt at dent repair by an amateur. The metal was surprising easy to move. Gives me a lot of confidence that the various glue pullers, pry bars, etc., can actually work.