I didn't try to cover the speaker holes on the GPS body because I was kind of concerned about heat build up. Those holes also vent the heat from inside the unit; I don't know how hot it gets inside the GPS, but like with most electronic devices, heat isn't their friend. So far, I've never had a problem with water getting into the unit under rainy conditions.
I did something similar with my power cable. I didn't want to use a data/power cable, since the data portion of the cable makes the unit reboot every time you cycle the power on and off. If I'm following a route in Trip Planner, the GPS reboots and I have to restart the route and figure out which waypoint I stopped at so I can restart the route from there. I got a male USB to male Mini USB cable, cut the cable and shorted out the two data wires, and the reconnected the two power wires. Now, when I shut off the power, the GPS just defaults to battery mode and maintains my position.
Plugging and unplugging a USB connector can also shorten its lifespan. My power cable stays plugged into the GPS almost all the time; when I remove the GPS from the bike, I take off the whole unit, RAM cradle and all. The only time I unplug the USB cable from the unit is when I have to program it, since I have to use a regular data cable for that.
I have a 12V-5V buck converter mounted on the left inside of my fairing, which provides power to two USB outlets mounted over the instrument cluster.
I wanted my power outlets to be up there by the GPS so I didn't have wires running all over the place. There are two Motopower USB outlets with waterproof covers mounted to an accessory shelf I built out of sheet metal scrap from a 20mm ammo can. The shelf bolts onto two unused holes on my windshield mount. It looks like this:
To keep the USB cable firmly connected in the outlet on the shelf, I built a little L-shaped bracket that I glued and zip tied to the USB connecter. The bracket has a hole in it that a screw can pass through. There's a matching L-shaped bracket on the top of each USB outlet, with a threaded hole. When I plug in the USB cable to the port, I screw the two brackets together with a long screw. It anchors the plug firmly in the outlet; it doesn't vibrate, and it doesn't come loose. The attachment system looks like this:
All the cabling stays up there near the windshield, and out of the way.