Marker Lights

I was patching up a crack in the flimsy plastic AC cover on the roof yesterday, making a temporary repair until I can locate a new one.  [Not topic drift, but context ]  When I read the additions to this post this morning, it occurred to me that you could make a gasket [just a cut out the shape of the bottom of the fixture] with Eternabond tape put it were you want  your light fixture and then mount the fixture on  a well attached but somewhat squishy surface.  

 No one mentions yet "Dicor Lap Sealant". 

+1 for butyl tape combined with mechanical fasteners. Conforms to surfaces, will compress into threads to help seal them, stays flexible, sticky and adheres very will but is not difficult to remove later when needed. Not messy. You can create little o-rings of it by rolling it into a little strings and placing it around fasteners. Some folks even make o-ring gaps by using a chamfer bit to open the edges of mounting holes a little. 

Silicone is often used wrong. Silicone works fine when sealed under something sort of like a liquid rubber gasket., but is a disaster as a caulk because its adhesive properties are crappy. Best practice is butyl putty tape under the lip of the lights, then 3m 4200 as caulk around the outside. 5200 not such a good idea around a consumable like a light that might someday need replacing.

I'm a contrary one, thus use 3M 5200, which I used to use as a bedding compound in my Newport 30 MkII.  That stuff will still be working for your grand children.