Infa Secure Vario is one of the narrowest boosters. Maxi Cosi Goliath is my pick for Convertible booster. It has also been released as a couple other names.

(from my excel file on car seat dimensions)
Infa Secure Vario Booster 4 y - 8 y 430mm width
Safe-n-Sound Maxi Rider Easy Adjust Convertible Booster 6 m - 8 y 7kg 460mm width
Maxi Cosi Goliath Convertible Booster 6 m - 8 y 5.1kg 450mm width