I'm not sure that fitting a notch filter to a hand scanner would a) be possible and b) effective. If the receiver is being swamped due to a wide front-end the only solution is a better designed set which, of course, is available if you want to spend money!!
Ground-plane antenna is a vertical quarter wave with several similar sized rods at the base, fanned out equally. Very simple to construct using an SO-239 socket. However, a vertical dipole is even easier as it just uses two wires and the entire antenna is vertical.