Look through/search the Installation forum. There's been plenty of posts about this style of mounting system. You'd need to drill through the sides of the cradle and use low clearance heads (so the empeg still can slide in).
The cradle normally mounts by sliding into a DIN sized opening, not falling through it due to the "lips" up front, and is secured from moving back out by bending out the tabs at the front, just behind the panel. Often a metal strap is screwed to the back of the cradle (M4 thread hole is there for, among others, this purpose) and somewhere inside the dash to stabilize the rear part of the cradle.
Edit:
Also check out the installation part of the
FAQ
/Michael