I wouldn't do anything other than crossover parts upgrade, because:
1. They sound great the way they are after cap upgrade.
2. Adding structure to the inside would be a pain, and change box volume and tuning. You are then messing with a great part of this system for it's size....the bass.
3.They don't need it. My pair is solid, without any major resonances, and lot's of fibreglass lining.