I would drill the hole diameter you need through the bumper. Then flip the bumper over to the back side. Now with a larger drill bit, drill a hole a little bit deeper than the nut. Now, when you attach the shock mount, the nut will be hidden in the bottom of the bumper where no one will see it. Once attached, trim the excess screw and fill the hole with epoxy.
Yep, you’re spot on, you drill through the bumper, into the middle of the cyclotron and screw it on from behind the cyclotron, its good and strong too, won’t budge at all. That’s what i did with mine.
You lose strength and support by not going through the cyclotron with the bolt.
It’s more work than is necessary to try and drill a larger hole halfway from the back without messing up the smaller front hole.
Because this is how the real packs had it:
About your shockmount. You could have gotten a nicer one from Exoray for about the same amount shipped. That one you got is decent but plastic and the edges are too “sharp”.
between 1/8" and 1/4". use the arms of your bumper in reference with the back edge of the shell to judge the space. the arms of the bumper should reach the back of the shell or come very close to it.