Once I get the outlets/switches in and cover plates fitted, I'll pull the cover plates and run a bead of clear silicone around the box extenders. I'll run a bead of grout caulk (probably gray or some earth tone) out the ridge the cover plate sits on and mash the cover plate into place. Once the grout caulk is dry, I'll come back with a knife and trim off any caulk that oozed onto the cover plate. That should help "weatherize" everything even though not much of this is exposed directly to the elements.