Gee, that Ascona convertible was popular.
"American Sunroof Corporation" made a prototyp for Opel. One could consider that a brand as well.
"Tropic" from Crailsheim made 2 prototypes, before going bankrupt.
"Hy-Tech", a company started by the same founder as Tropic made at least one other prototyp and maybe a handfull of actual production cars.
Jankel also made a convertible, for Vauxhall.
Envemo apparently made some 10 convertibles in Uruguay
Sulam of Brazil did the same apparently.
By the way, was the Opel Monza mentioned?