I've actually given this a lot of thought, though I've yet to actually BUY
a GPS. Due to the problem of downhaul lines on the mast base area and visibility problems of attaching it to either cross bars or lacings, I think a good alternative would be to buy/fabricate a fore arm band to hold the device, something like a Walkman holder for the upper are, but down lower.
The reason that I think this will work is that I have a wrist compass that I use very successfully over my wetsuit about two to three inches above my right wrist. I've gotten used to orienting and using recipricals to determine my compass course and this may or may not be an issue with a GPS, but it's a pretty small matter to cock your arm parallel to the direction of travel for an instant or two to get a reading. I also made a very functional handheld VHF radio harness to attach to my PFD using an existing waterproof radio bag/case and sticky-sided velcro. No reason that the same type of materials, pre-made or your customized, couldn't be employed for your application. Good luck