If you are interested in adding a Submissive to your Stereomour, it will be much, much, much simpler to build it in its own chassis.
To answer your specific questions: the switches mount on a sub-chassis that is 15cm wide. The highest point on the sub-chassis away from the main chassis is just under 4cm (that would be for standoffs, sub-chassis, terminal strips. More space than that would be better). Count on AT LEAST 5cm between the center of the switch holes and any other components, again for terminal strips.
The overlay is white text on black. You will need to fabricate your own to put it in a Stereomour, for a variety of reasons. First, if you are planning on reusing any existing holes, you will need to add two holes, as they are differently spaced (each switch hole is about 7.5cm apart on the Submissive, 9.5cm between the pot and the selector on the Steremour). Additionally, there are slots in the plastic that will most likely show the unused holes in your chassis. Finally, the selector switch is on the right on Submissive, but on the left on a Stereomour. To use the plastic plate would require moving the selector switch to the other side, thus require re-wiring all the inputs.
All in all, it would be a major undertaking to graft this onto a Stereomour. The design intent of the Submissive is to make it simple to add this attenuator to anything. If you buy one, you will get a well thought out chassis designed for the express purpose of serving this attenuator design. Simply plug it into an input on your Stereomour and turn up the pot. Also remember that down the line, you might want to use this attenuator with a different amp. Or sell your Stereomour, or sell your Submissive, or any number of different possible scenarios. The separate chassis will make those decisions not only simpler, but feasible.
