Take it off, put it behind the passenger seat, radio works pretty good without it and only slightly better with it, been driving like this for a year+ because I visit so many customers in their underground parking.
I don't think a shark fin antenna will fit due to the curvature of the roof, plus they look silly!
If I can recall correctly the antenna off the chevy spark or sonic has the same thread pitch as the PMC antenna and will screw in, its flexible unlike the stubby antennas and possibly cheaper.