I have a job that I need a large weatherproof in-wall box to hold the Fios nic/ demarc equipment & power supply. Owner doesn't want it hang on the wall......
Looks like the depth needs to be about 6".
When we can get the drop to an inside location we use a 36" Open House enclosure with a hinged door. The Fios "tech" can use the inside unit and it fits pretty clean inside. There isn't much room to spare though.

Hoffman is the only rugged outdoor enclosure in steel that I could think of right now.

We usually have the/a carpenter build an enclosure. It's normally pretty simple to cut the wall open, head out the opening, frame edges that extend from the wall the necessary depth, and then put doors on the front with weather stripping. Often they can reuse the original siding. The Fios ONT comes in both an indoor and a weather rated enclosure. If you can fit the weather cover (that the normal ONT fits into) into a semi decently closed space, it will weather great.

I'd also try walking into an independent electrical supply house and describing what you need, there are tons of specialized high voltage enclosures that can be bought empty.

I'm not sure if the VZ tech will install the battery backup outside, IIRC they require it to be installed in a temp controlled environment.