I am also thinking about how to mount my Garmin 255W GPS. I will be ordering the RAM Aqua Box and Gilsson Direct Wire Cable from Amazon.com (best prices found so far). The medium-wide RAM Aqua Box is the right size for both our Garmins. The cable has a transformer to allow direct connection to bike power with isolation for safety to the GPS electronics. There is a less $$$ cable that does the same but that one has a straight mini-USB connector and there would not be room in the Aqua Box for it. The Gilsson has an angled connector just like the Garmin car connector. I already have a fused Aux power panel in my left glove box to connect this to that is only powered when the bike is on.
[ame="http://www.amazon.com/gp/product/B001RTOSRY/ref=s9_simh_gw_p23_i1?pf_rd_m=ATVPDKIKX0DER&pf_rd_s=center-3&pf_rd_r=1QWY41SNXS2DC343055H&pf_rd_t=101&pf_rd_p=470938811&pf_rd_i=507846"]medium-wide Aqua Box
Gilsson Direct Wire Cable (warning on this product- I ordered and received it and THEN found out this direct cable module does not support the Garmin wide screen Nuvi 255W. It will, however, support the regular screen sized Nuvi 255.)
Then I plan to make my own bracket to mount on the dash similar to how Hound did it. Actually, Hound has a very close variation on what I was thinking of doing on my ST1100. Will be watching this thread for any other ideas that would be worth considering before I finish mine.
What model ST do you have?