There are many ways you can mount a rear motorcycle fender, this is just one of them. I wanted to make strong mounts as the .125" aluminum fender needs to support the weight of a passenger as that is where the rider sits on this purpose-built race bike.
I wanted to give ample clearance between the fender and tire, so I used a piece of 1" rubber garden hose, cut and taped to the tire so it would give equal clearance all the way around. Also, I moved the tire forward in the axle slots so that I would never have any issues when adjusting the wheel to take up chain slack as the chain wears.
I set the fender onto the tire after radiusing it to fit just right (see the How To Radius a Motorcycle Fender DIY Tech Tip). I then cut a piece of 1" tubing, the same .120 wall DOM tubing used to make the hardtail frame, and coped one end of it to fit onto the existing cross member. I did this a simple, quick and dirty way by clamping the tubing in a vice and drilling through it with a 1" step bit. I then cut it to the proper length to be a tight fit against the fender.
I took a hair off the length at a time using a lathe, but you can use a grinder to get it in the ballpark then use a hand file to get the proper length. I then used a threaded steel bung with a .75" outside diameter that was tapped and threaded for a 3/8"-16 bolt and chamfered the edge of it.
The bung is a nice sliding fit into the tubing, and I then TIG welded it up, just fusing it with no filler rod. You can also MIG weld it up, of course. The benefit of having a lathe once again reared it's head as I chucked the piece back up and faced it, taking off just enough material to give a nice finished surface.
Once you tack your first fender support in place, you can carefully mark and drill your fender and bolt it in place. Then follow the same steps to fabricate your bottom mount, again carefully lining up the fender before drilling. Now all that is left to do is make some fender struts to finish the sturdy mounting of our fender.