Plan is indeed to have a router and a PoE adapter in there for the security cameras.
The problem with these and that enclosure is heat. With the electronics in there, things will get really toasty. Even if you don't have a fire hazard, the life of the PoE adapter and router could be degraded if you keep things perfectly dust free. (Those devices
need air circulation.)
I'd suggest allowing for air circulation with vents and using air filter material to keep the dust out, instead. Some dust will still get in - but the question is, "Will the heat or the dust be more detrimental to the equipment?"
I used this enclosure for this type of thing:
http://www.centralvacuumstores.com/ms-h312-kit-structured-wiring-enclosure
It's nice in that it mounts in between the studs, has a vented cover, and has knockouts for an electrical/outlet box in the bottom. (The box mounts to the outside of the box.) If you want to still use the box you purchased, you should be able to mimic the same method - cut a hole in the side and then mount a proper old work electrical box through that hole, so the outlet is available to the equipment inside.