According to the ESB cast and crew list, there were only 3 sculptors (modellers) hired to work in the preproduction of the movie (source ==>
Star Wars: Episode V - The Empire Strikes Back (1980) - Full Cast & Crew):
From the
Star Wars - The Blueprints book (quote taken from the first post of this thread):
It says
the outfit complete with backpack and helmet range finder, so I guess it was sculpted (at least partially?) by Jan Stevens with the aid of Brian Archer. It is possible that the jetpack was build (with clay) around the main tube and the side tanks and possible other scratch made parts.