Or get a used stock 2 part seat with rider and pillion or just the Rider seat - dirt cheap and very light to post.
Utopia make 2 versions of rider back rest mount for Triumph Rockets:
one for single headlight 2008-2018 RIII Tourings one piece dual seats like Joemoe's (Paul) that require cutting into the cushion.
They also use this mounting system for the dual headlight 2004-2018 Rocket IIIs one piece dual seats.
They also do a version with a mount that bolts to the separate pillion cushion mount on the dual headlight 2004-2018 Rocket IIIs with the stock 2 piece Rider and Pillion seats.