Putty pads are a very typical way to seal off outlet boxes and are part of many a theater build. Curiously, though, the instructions I found for using them were sparse at best and non-existent at worst. I got the impression that it should be "obvious" how to install them. Well... it wasn't for me, and it might not be for others. I finally got a method that worked well for me after maybe four outlet boxes and figured that I'd describe that method here.
I make no guarantee that this is the "right" way to do it. In fact, if I'm clearly doing something wrong, don't hesitate to correct me.
My outlet boxes are standard Carlon single-gang adjustable boxes, which are somewhat ubiquitous in this application. I needed 1-1/4" clearance for the two layers of 5/8" drywall that will be installed soon.
The putty pads came from the Soundproofing Company at roughly $5 each. That's a pretty standard price for these. I don't see any brand name associated with them. Hilti makes their own brand of putty pads, which may be available locally in some areas. You may need to buy them in sets of 20, though, which can get expensive. The putty pads I used had a plastic release sheet on each side. Hilti pads appear to be individually packaged. I've seen some suggestions that using readily available "duct seal putty" (found at the big box stores) can also work for quite a bit less money, but I didn't try that.
I didn't see this step anywhere, but I noticed that a single square putty pad would leave one side of my outlet extension (the part that extends past the stud) completely uncovered. That may not matter, but since there is physically enough material to cover the entire thing, I made a little effort to do so.
I started by cutting of just over an inch of the putty pad, while it was still covered by the plastic on both sides:
I then removed the plastic from both sides of my cut-off hunk and wrapped it around the extension, making sure to primarily cover the "other" side that won't be covered by the main pad. The pad is quite sticky and moldable, so it easily adheres to the box:
Applying the Putty Pad
Covering a three-dimensional object with a two-dimensional pad would normally mean bunching up the pad in the corners quite a bit. I got around this by cutting some slits in the pad to allow me to go around the corners easier. The slits are roughly the depth of the outlet box in and the width of the outlet box high... but I just eyeballed it, since it's not critically important:
That was done with both sides of the plastic still on. I immediately followed this by removing the plastic from one side. I then pressed the pad against the side of the outlet box away from the stud, so that the pad was flush with the front of the box:
This is the time to remove the remaining plastic! Yes, it'll make the manipulating it a lot stickier than if you kept the plastic on, but in my experience with my first guinea pig boxes, it was extremely difficult to remove the plastic once the pad was molded in place.
The top and bottom are simply folded over and pressed in place. The back is also mostly just folded over. The tricky part is to them take the remaining flaps and to mold them into the holes in the back of the box:
The most important part of all of this is to make sure that the cable entrance into the outlet box is fully covered. I accomplished this by wrapping the putty around the cables first, and then pushing the hole mess up into the hole. It covers it up very well:
You may note the exposed area on the bottom of the outlet box. I'm pretty sure that doesn't matter. It is notable that the putty wasn't as stretchy as I thought it would be, based on a video I saw for the Hilti pads. They do stretch a little, but not a lot so don't count on that.
And that's it. I tested the outlets by shining a powerful flashlight from below, behind, and above the box and looked for any light to show through. I also shined it into the box and looked for any light leakage coming out the back. I'm glad I took this step because three of my earlier boxes did have some tiny holes that weren't visible at a glance but were clearly there with the light. Fixing those was easily done by just molding the putty over the discovered holes.