Exterior sconce, install details – GreenBuildingAdvisor

Stumped on how to install exterior downlights, see attached sketch & photos.

Wall buildup from inside to outside:

– 2×6 framing

– 1/2″ Zip sheathing

– 1 1/2″ Rockwool

– 1 1/2″ Rockwool, seams staggered

– 3/4″ vertical furring strips

– 7/8″ horizontal shiplap, western red cedar

Downlight is ±3 3/4″ wide * 3 3/4″ tall, and I’d like to center it on the shiplap.

The back of the fixture pops off with a couple set screws.

The fixture back has 4 mounting screws to get mounted to whatever surface. The fixture’s wire pops out the backplate through a gasketed hole. I haven’t been able to find a junction box small enough to not stick out past the fixture edges, nor one on which the back plate’s mounting holes align. I was going to block out a mounting block, sloped on top, and liquid flash the hell out of it to run the wire through.

It looks like it’d be a real tight fit trying to get a piece of 12g Romex + wire nuts in the fixture with the back plate installed; nevermind the fact that the gasketed hole isn’t big enough. Seems like it was intended to be installed over a junction box of some kind?

I could maybe drill a hole in the piece of siding, run the fixture wire through, mount the backplate, mount the back plate to the piece of siding, temporarily hang it a bit lower than necessary, insert a junction box into a hole in the mounting block, wire the fixture to the Romex in there, and carefully push the siding into place. This seems unnecessarily complex, and would mean removing siding if I ever had to replace the fixture. I could caulk the top and sides of the fixture to the siding, but that seems like it leaves the open j-box vulnerable.

Am I overthinking this? Do I just need a 3″ pancake box (haven’t found one but surely they make tiny pancake boxes) inset into the shiplap?