You've got a few options.
When I went through this, the solution came to me when I considered where I wanted the hot air to be exhausted: not out the front, not the sides, but the back or underside would be ok. The top was definitely out, because I suffer from coffee-cup-spill-o-phobia
It was also relatively simple for me to select the final location, because I simply pulled out the rear panel, which normally holds the PSU, for a perfect fit of a 6" by 8" opening (no cutting required!!!), since my PSUs are mounted up front.