As I understand it the Carbonite map simply can not be any shape other than what it is.
As far as Spartan UI goes, there was a modified version that had a square map opening. If I remember correctly, it was meant for Carbonite (or any other square minimap mod). It's an old version and I'm sure it's no longer available. Perhaps you could modify the graphics yourself.
Ahhhh, the vagueries of the aging mind. Wait.... What was I saying?
Carbonite <----- GitHub main module (Maps ONLY) download link. The other modules are also available on GitHub. Carbonite-CLASSIC<----- GitHub link to Carbonite Classic. Thanks to ircdirk for this!