Here's a 'glue in' solution that will work for your needs. I've been meaning to post this for a while, so this looks like a good place to do it.
I had similar problems-
1. USB connection too loose,
2. USB connection not waterproof
3. need to be able to remove the unit from the bike and plug it into a computer or into my car.
Here's what I did:
1. to tighten up the connection cut a small sliver of plastic from a cheap bottle of water, bend it 90 degrees and slide it into place as a shim between the Nuvi case and the USB connector. Tape it in place ( If you squint you can see my duct tape down the side of the Nuvi case). This will take up all the wiggle room and keep the connector tight. [Edit: now that I think more about it- I think the little piece of plastic actually went right into where the USB plugs in- that would make more sense. This has been a while, so you'll have to figure this one out!]
2. Go here: http://www.usbfirewire.com/uextendminib.html
and get a 4" "Up angle, mini B USB extension" $12. (Check the pictures on the web site to make sure it's up angle and not down angle- it's been a while since I ordered and with mine glued in I can't see it anymore.)
3. Slather the connections with dialetric grease, plug it into the back of the nuvi and then glue it in with RTV, sliver of plastic and all.
4. Make a rain shroud for the other end: take a large diameter piece of shrink tube and shrink it down over the wire and connector at the other end, making sure not to shrink the part where you will have to plug in your hardwire from the bike. I glopped some more RTV where appropriate.
5.Modify the RAM cradle so that you can rotate the device in and out with the extension attached. I used a dremel. It looks like it would leave the cradle very weak but actually that part does very little for holding in the Garmin.
Voila! The finished product. Rough looking, but now with a waterproof, vibration free connection and the portability you want.
Works for me anyway.