For a proper fit, the side of the extension should have a flat area in the mounting position at least 33mm long by 12mm high. Things can be tight in the Eb position, but it is OK to grind a bit of height from the bottom to make it fit.
Once the proper position for the bracket is determined, drill pilot holes with the supplied 3.3mm bit at the midpoint of the slots, about 24mm apart. The default supplied screws are gold-plated #6 truss-head self-tapping stainless steel. Use only a fresh, high quality, #2 Phillips driver, and pre-form the threads with the included un-plated screw.
The standard Capo and E-Stop arms fit extensions that measure up to 3/4" (19mm) from the mounting surface to the string center. If you have an unusually wide extension, let me know, and I can ship longer fingers. If an unusually narrow one, then ask me about stand-off spacers.
Brass screws are now available for the purists out there! Slotted or Phillips. You have to tap an 8-32 machine thread though. I have the tap and pilot drill if you need it.
When working with a narrow extension, or if more standoff is desired for any reason, you can get pre-cut spacers in various thicknesses.