Microscale Micro Crystal Clear. Goes on Similar to White glue, dries clear (if thick it may take a few days to fully clear out). It stays a bit flexible, so if you needs to pull the LED out, it would come out. Only issue is that its not an instant bond, so you need to get the LED to sit still until it tacks up.
Don't use canopy glue though, that needs a reaction with the plastic to work, and whatever resin LEDs are made from doesnt react to it.
Regarding hot glue, the trick is to put the LED into the headlight lens, then glue the back side to lock it in place after the LED is installed. Sometimes its hard to get the glue gun in there, and it makes a stringy mess too.
