In that case I would off center the tweeter to get it out of the direct path of resonances from the woofer, as you've got about 10 inches center to center.
Going aperiodic will lower the impedance hump, and reduce the tendency for amplifiers to over-damp the woofer because of room boundaries feeding back at it, because the aperiodic vent will bleed off resonances that are too stiff and cause this reaction. Well I'm thinking it'll half them, anyways.
I suppose you could put the woofer and tweeter down so there was an open space at the top. It would look strange but it might help keep the woofer out of a resonance node that will also tend to damp it more than desired.