Hey there, I did put 59 caddy lights on my 1500. To answer your initial question, if I understand it- on my original attempt, I just mounted the "cups" for caddy lights to the Kaw light backing, using the original holes for the stock lens (not sure if I used over-sized screws) -- I think that's what you're asking. They held alright.
I did not really like the extra weight of the caddy lights on the stock light rubber stems. I originally made brackets to help hold them up. However, my second round, I switched to HD lights, and hard-mounted them, and made aluminum spacers to fit the caddy light to the light housing. Here is the write up:
https://www.vulcandrifterriders.com/mods/Caddielights/writeup75.pdfHere's the page with several different folk's versions....
https://www.vulcandrifterriders.com/caddytaillight.htmlIf you have any questions, I'd be happy to help you if I can.