if u want to use series structure u may have problems due to signal integrity/EMI issues!
the best solution depends on the required output power:
1. for lower than 150 Watts the best solution is flyback, it is simple, easy to drive/control ,fast and also there is no need to large output inductor.
2. for higher power forward converters are attractive. they are constructed in several topologies like single switch forward, dual switch forward, half bridge and full bridge; by the way the most efficient/compact method for your application (low volage (12V) at input) full bridge is the best and after that dual switch forward converter.
BEST!