I got a Trapezohedron on my third Adamantoise. Lucky me
Damn... you really are lucky. I've killed about 30 of those and still not Trap.
You can triplicate your original Trap by creating Vanille's Nirvana weapon (it would cost you 1.5million gil) and then dismanteling it. By dismanteling Nirvana you get 3 Traps.