Don't. It's a really bad idea for several reasons.
The rear case has reinforcement around the factory screw hole, but not elsewhere. Any modded belt clip is likely to be LESS secure than the factory clip, not more secure.
The rear circuit board is very close to the rear case. If you drill holes without completely disassembling the scanner, the odds are better than even that you will hit the circuit board with the tip of the drill when you break through, or worse yet, screw into the circuit board when you install the screws. Either scenario will result in expensive repairs not covered by warranty.
There are several cases that have integral belt clips that are stronger than anything you could attach to the thin plastic of the rear case, which also offer significant protection to the entire scanner, and have zero risk of screws or drills bamaging the rear circuit board. You are much better off selecting one of them, rather than trying to attach a non-factory belt clip.