The switches from 4x4mods fit perfect in the knock out holes in the X. I have one form them and it's great. It's also cheaper than the Contora switches which also fit (but don't have the cool color selection and pictures on them).
I think you want pin 87 not 87A going back to the locker. I beleive hooked up to 87A will result in the switch working just as it does now. 87A should be powered when the relay is powered which not what you want. I would have to go back and look at the schematic for the relay, but I think that is right. Other than that...everything else looks good. I can't remember the colors of the wire, but basically you want to disconnect the wire from the switch that goes back to the locker and place the relay inline between the switch and the locker.