I have a 60mm Apexi EL series boost guage that i stuck in one of the vents too... but i stuck it in the one next to the clock. Anyway, there shouldn't be any cutting required, once you take off the dash you'll find that the vent is actually 2 parts. One is a ring in which the "turny" thing sits.
What you do is pull out the whole unit, and remove the centre part from the ring, it shouldn't be hard... just pull it out. Next, if your guage is a 60mm or smaller guage, it'll be too small and won't fit properly. What you do is wrap the guage in some foam, the stuff you can get as a tape. Anyway, stick it on so it creates a tight fit against the ring. Black will be the best colour b/c you'll be able to see a bit of it after the dash is back on. Anyway, once you fit it into the ring, put the ring back onto the dash. Reinstall, and voila.. you're done. Oh, i forgot to mention you'll need want to seal it so you don't get air blowing out from the vent still. I cut out the top part of a water bottle and some thermal tape so it won't come off when i used heat.
I suggest you put it int the one next the clock... when it's really hot or cold, the one closest to the door works better as a vent. There's a lof of piping to get to the one by the clock, that and the steering wheel kind of blocks the view to the other vent...IMO
(oh, and if you do choose to put it by the clock, you'll need to pull out a bit of "flat" piping, it won't be hard.. you'll figure out as you do it.. i did
