If you have a tuorpack it would be a good place to install them there and run the wire to the speaker pods through the bass holes
Yeah that - You have a 2013 ultra if I remember correctly. Use double sided tape and mount them under the flap in between the speakers on the tour pak.
Potential exposure to water outside the tour pak shouldn't be an issue?
No Sir! 1. the leather flap is protecting the crossovers. 2. The crossovers themselves are insulated pretty well from the factory. Keep in mind the Polk MM651 speakers are Marine Certified speakers actually designed for boats. I ran mine through torrential downpours and nary an issue with the crossovers. The crossovers and speakers are solid in regard to rain.
If they are in fact the Polks, I just threw my crossovers in the pods. Tucked them toward the back and packed with polyfil to keep them from rattling around and rubbing on the speaker.