Mace (Reach 1, Str/2+3P, AP -, 120Y)
Custom Look - Level 2: 1 slot, 1,000Y
Personalized Grip: 1 slot, 100Y
Propulsion System - Flight: 8 slots, 20,000Y
Total: 21, 220Y (and the potential loophole that Vehicles and
Firearms have listed slot maximums)
Though, given it's not even remotely broken, your friendly neighborhood GM with a sense of humor should love it.