You could also put the filter not at the antenna, but back on the lead
in just before the receiver. That way any 106.9 being pick up in the
feed line are also attenuated.

Have you tried calling the 106.9 station? Years back, many stations
freely (or very cheaply) provided filters to people experiencing
interference from their signal. I remember back in 1987 I had a customer
whose Muzak SCA reception was being trashed from a nearby broadcaster. I
was expecting to pay for a filter, but the station engineer literally
left one at their reception desk for pickup.

I would not expect free, but you could hope for inexpensive. Never hurts
to call.

Good luck,
B.