Work in progress!
This is a 2 switch box that mounts on top of the tower using the existing screws or similarly sized M3 hardware. I used it to hook up switches to my LEDs and part fan so that I could disable them as needed without having to disconnect JST connectors or something similar.
The switches I made it for are "SPST Mini" toggle switches that I got at Radio Shack. There's a slot for the little guide washer so they stay straight. To fit them in the box I had to remove the bottom-most nut on the switch assembly, the one intended to be secured inside the box. I set it up so that I could wire up and secure the switches with the backplate removed, then tuck the wires in through the slot at the side until they were in the dome cutout, then slide the backplate on to secure it all.
The holes at the top line up with the tower screws so you can use a small screwdriver or allen wrench to secure it. I used M3 screws and an allen wrench. My screw holes didn't line up perfectly and I think I drilled one out a little bit to make it fit nicely. I don't remember if I fixed this STL yet. Edit: I printed the bottom layer by itself and the holes matched up with my screws, so I must have fixed it. But let me know if anyone has any trouble with it.
The back plate was a little tight too, I had to file it down some to get it on.
I didn't like how the screw access holes in the top just sat open like that so I made a little bulb and fan icon to stick in them. In my picture I colored the upper ridges with a sharpie, but you could also do a filament switch to achieve the same thing.
Will upload new versions as I fix the little things wrong that I mentioned above.