Is there supposed to be any sort of clamp or band around the innermost shifter boot to hold it into place, or is it just a snug fit alone that does it?

Mine had a zip-tie around it, but it might have been from the previous owner.

No clamps. There is a lip on the trans that it fits over. Fun to install, an exercise in strong finger manipulation and stretching in an awkward spot.

Grab with long nose plier from below and pull it down in two or three places. I then secure with a zip-tie. The rubber tends to soften over time, and loosen up. I don't think Nissan specified a clamp there.

Thanks guys. I had looked through the manual and the fische and didn't see any retainer of any sort, but you never know.

The zip-tie sounds like a great idea while I'm in there though!