Thanks for all the advice guys, and saving me machine shop time trying to build a custom bracket.
I may buy two more Foretrex's to mount on the boom, but that seems a little unnecessary if I have one on my wrist.
I scored a deal on some AquaPacks (5 for $20, they usually cost $20 a piece) that are specifically designed for VHF radios, but also fit the GPSMap.
If you use something like this you have the option of velcro strapping it in many places (body and boat).
Aquapac pro 
