My bike did not have front turn signals on it, so I got a bit of aluminum bar stock (1" by 1/8" thick), some aftermarket front turn signals, and fashioned a U-shaped bracket to bolt onto the forks, underneath the horn bracket. Chances are if you search for some of my VERY EARLY postings, there might be a picture of it on our forum.
I'm guessing you can do the same, finding some way to bolt it onto the frame under your seat. Maybe make 2 L shaped brackets to bolt in where the tops of the shocks attach to the frame?
Granted, something well formed, welded, and painted and stuff would be the way to go, but if you just want to obey the letter of the law (thou shalt have rear turn signals), either way would work.
I hope this helps.
Luke M
Used to have a 1979 CB750L, sold it as a parts bike, now riding a slightly modified 1984 VT700C. Network/Field Engineer. Central OH, USA, Earth, Sol System, Milky Way Galaxy, Universe.