I was asked about this some time ago...

I lifted my compass out, and inserted the power cable for my GPS through the hole, and re-installed the compass over the cable. I tightened the screws, and used some lightweight tub/tile caulk to seal the lift from the compass bezel. It worked well, and looks nice, and best part, is no cutting/mods to dash. The GPS holder is mounted with that super strong plastic Velco from 3M you can get at Boater's World/West Marine.

Pics:

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All wired in...

Its a 26' Sonic, and I have flown the boat. Nothing has come loose, just had the GPS lay back once.

The plastic Velco is STRONG. I had to pry the base off with a screwdriver.

I have a Garmin GPS Map 175 that I installed in my boat much the same way, although I though bolted the mount. I normally keep the GPS out of the boat since it doesn't have to take the daily abuse when I really don't need it. There have been times when the GPS came out of the mount from a launch. Need to keep the strap attached to something so it doesn't fly out of the boat.

One more shot. The GPS is a Garmin, and the unit locks into the base.