I'm not partial to chopped rear fenders, but I did want to get rid of the stock Harley tail light on my 2006 Sporty.
While this isn’t a beginner project, if you take your time and perform each step carefully, you should get decent results.
The first thing I did was to remove the tail light and disconnect the wiring from the junction plate. I decided to keep the junction plate intact, rather than cut and splice the existing wiring, to make it easier to add a new tail light in the future if I want to.
Here’s what it looks like after complete removal:



The mounting tab has to go, so I pulled out the dremel with a cut off wheel and carefully cut it away. I then used some 80 grit on a 6 inch wheel to clean up the whole area:





Since I am going to be brazing pieces of 20 gauge steel inside, the rubber channel needs to be peeled back about a foot to protect it from the heat. I did this carefully with a single edged razor blade:



I then took my drill and a circular pad with 80 grit, and cleaned up the inside:



I cut two pieces of 20 gauge steel, one to cover the mounting holes, one for the tail light rectangle, and brazed them in place:



The plate that covers the rectangle has a mounting tab on it with a quarter inch hole in it to mount the original electrical junction block. I then sprayed some primer to tidy it up:



I glued the wiring channel back down and mounted the contact board:



I reinstalled the fender and started using a steel reinforced filler. 4 coats were required to prep the surface correctly.

You need to put light layers on to prevent any voids from forming. I block sanded with 80 grit between the first two layers. By the time the third layer was on I had a pretty decent surface – and could see the existing low spots that needed more filler.

Use a long sanding block to shape the filler. The area that used to be occupied by the tail light is actually quite flat, and needs to be built up a bit to contour properly with the fender above it:



Two coats of primer and I am done for now. It is over 108 here in the desert and way too hot to paint: