I actually have 2 kinds on my panel:
30mm OBSN (they screw in from underneath)
24mm OBSF (they pop in securely with side tabs for tension)
To get the 30mm buttons in you need to drill a 30mm hole halfway from the top until only the center of the drill bit pops out of the bottom. Then you flip it over and drill from the bottom with a larger bit (lining up the drill bit with the center of the hole). (I can't recall what size the bigger drill bit is, but I'll look when I get home). This allows the button to be placed on the top and the larger ring to be screwed on from the bottom.

As an experiment, I used 24mm buttons for my admin buttons and they were pretty snug in each hole. You can probably get away with using the OBSF 30mm buttons which just pop in and out, just like the 24mm ones. But this other method I mentioned above was so easy I'd just go with that.
Hope this helps!
DeLuSioNaL29