or be very careful. first time i fiddled with the horn mounting on my Jensen i poked a hole in the dust screen for the tweeter. It's about two inches from the diaphragm of the tweeter and the pase plug is in the way but still. Any non magnetic screwdriver would suffice, i wouldn't trust torque you can get with a plastic srewdriver.
in any case, are you missing the tweeter or just the horn? If it's both then you might list them as a "win this one and you can pick up the other for a %age" type auction and leave it up to the winner. Do they match at all(cones, ser. no.s)? If not you may as well list them singularly and the winner can find thier own tweeter solution.