Ultimate seat backrest

[LeaderDuece]

Those are the same dimensions on my Mustang backrest. So maybe you can search for one of those as well. I’ll try to get a pic.

Or if we give you some pics, you could get a cheap backrest and make the inset for it.

[RoadStarRaider]

[umpire3]

I've got this one on my bike.  The mount is bolted to the front seats' hold down.  The tongue for the back rest comes around the back of the riders' seat.  The back rest itself attaches to the tongue.

https://www.ebay.com/itm/Yamaha-Stratoliner-Roadliner-Raider-Motorcycle-Driver-Backrest-Quick-Release/222436580159?hash=item33ca40db3f:m:mWPVV-X_p8OX03vTszQ-C8g