That mount looks like it is going to be pretty solid, if you find that it is digging into the foam you can always add nylon spacers between the fixture & cover. Those are really neat looking fixtures, are they a cast metal or plastic?As it turned out, good I hadn't glued originally as my spacing for the braces for the scones accommodated the needed gap to feed the grounded plug through, but not the spacing for the outlet cover I'm using to mount the fixture to - so both had to be spaced out a bit further. Which then meant the matched brace on the other two got moved too (if I ever want/need cross bracing or mods to support the rebar or a sign upright, I wanted easy ability to put in cross braces between the side bracing later).
My fixtures are inexpensive options from Amazon, and the back plate is narrower than a fixture box, and the included mounting hardware are wood screws - definitely not intended to mount to a fixture box. So, after lots of considerations for how to mount so that I can remove for storage, I opted to use an outlet cover. The cover will mount to the bracing, and the light will mount to the cover. I can thread screws through the fixture plate in into the threaded holes in the plate. For redundancy, I can use a box mounting bracket on the back side and some wing nuts.
View attachment 21747
This is the fixture from Amazon. I'm gong to use a flicker bulb. My cemetery has a very gothic style to it and the open diamond pattern in these match well with the finials and gate toppers.
View attachment 21748