How big a hole do I need?

[herringchoker]

Or you could use a 4" rubber tractor utility lamp housing...just replace the clear seal beam with the 4" red stop/tail seal beam. I used these for years for tail lights on dump trucks & highway tractors. They take quite a bump and just bounce back. Like this:
Grote Industries: Product #64931 - w/ 12 Volt, Sealed Beam

 

[wushaw]

If you have a 6" dial caliper or regular caliper then you can measure it and drill from there. Cheap ones can be had at the big box stores General makes them. Calipers - General Tools & Instruments

 

[pat32rf]

Thanx Wolfpack; I have used these on another trailer but I just used silicone to hold them into the pipe guard. This time I want to do it right. I actually have two sets of these, one is LED and the other a used set of incandescent from another old trailer I scrapped. I have pretty well every adverse condition to deal with up here. Salt and snow in the winter, vibration on gravel roads, and at least once or twice a year the trailer gets submerged at a loading ramp or beaver pond. The regular fixtures last about two years before they give trouble but the sealed units seem to last forever.