Polyfill is going to dampen the inside of the enclosure also. However, putting polyfill in a ported box will result in you having polyfill all over the inside of your trunk.
If you go with a larger enclosure, why raise the tuning? If you're just after a higher score, that might help you out. However, increasing the enclosure volume will decrease the power handling of the system. Tuning extremely low (25 Hz) is kind of pointless in a vehicle because you've got cabin gain helping you out so much.
Why do you want to build different enclosures? Better scores?