208 VAC usually comes from one floating side of a Delta 3 phase transformer. 240 VAC usually comes from a grounded center tapped transformer. Using an autotransformer would be very hazardous and violate code. Probably tripping the 208 VAC breakers.
The only way to do it correctly would be with a transformer with a 208 VAC floating primary and a 240 VAC grounded center tapped secondary.
That being said I think a buck boost transformer will work.