So I installed the speakers this morning.
(Click image to enlarge)
Rather than try to figure out a way to keep the crossover anchored inside the door somehow, I realized there was enough room right next to the speaker on the inner door itself. So I had just enough length on the factory speaker wire to poke it through the foam door "skin" (for lack of a better word). I ran the crossover cord between door and the speaker mounting adapter. I used heavy-duty velcro to mount the crossover itself right next to the speaker. The tweeters are on a swivel so they can be aimed up and they have a 0db/-3db button to tone them down if so desired.
These speakers blow away the Polk DB 5x7 coaxials they replaced. Very happy with the sound.

(Click images to enlarge)