What part of East Tn. are you in?

 

Here's a couple of pics of that same setup that I recently took off. It's the stock streetglide fender with the same extension you have. I hogged the extension holes out so that it could be removed easily. You could just clamp it into place and drill a couple holes through the extension and fender and bolt it up. I have also heard of people using 2 sided 3m tape. Hope this helps.

 

I mocked mine up by taping up each side of the fender, then mocked the extension by using tape to hold it in place. Marked the holes with a marker. Removed extension. I staked the marks with a nice sharp"automatic" hole punch to make a divot so the drill bit wouldn't walk on me. Do not use a hammer and punch unless you have something to brace the fender on the way inside.
I then started by drilling out the holes in stages. Drilling a small hole and then working my way up to a size slightly larger than the bolt I was using to allow sight adjustment before tightening the bolt and nut.