Waterproofing tail lights.

[1964Chevy]

Howdy all. I have a 64 Chevy short step, I'm replacing my original tail lights with flush mount LED's, I plan on mounting them on a bracket about 12" above the original lights. My prob is that I cannot find how to attach Stainless flex wire covers to the back of my flush mount lights (I am going to build a backing plate to secure the lights to) that will prevent water from getting to the back of the lights.
LMC offers rubber wire boot covers, and they also have the stainless cable covers, but I can't figure how to attach it to the back of the light.
Any help would be appreciated.
This is a great forum, thanks for all the great info.
Jerry