ha,
its a question thats as old as time it's self. Officially no, I did think about trying to modify the OEM Triumph one, but it's $$$ to get wrong.
As far as I know \ thought about there are 2 options -
Fast eddy's
http://www.fasteddysports.com/?page_id=3&product_id=413 is for the driver but I reckon if you moved it back a bolt hole or made one your self by doing this.
Buy a backrest only, Get a bit of flat bar and attached it to the two back bolts, then welded a bit of tubing or square bar (like in the fast eddy link), off setting it to fit the backrest and chrome it.
Its subtle enough not to notice. The tubing or square bar would have a bolt or clip that you could slide a sissy bar back rest into and easily slide back out, taking away the backrest and ugley look. I reckon it would take about an hour to make, get your local shop to weld it. The back rest should just slot in and fix in place.
Second option is the kuryakyn plug-n-play-universal-sissy-bar-mount. Another member of this site used this but said that it due to the angle the backrest tilted back to much
https://www.denniskirk.com/kuryakyn/plug-n-play-universal-sissy-bar-mount.p491548.prd
