For waymarking routes and tracks I have been using my Garmin Etrex Summit GPS receiver. I bought it to keep in my bag when out in the winter hills so that I could always obtain an accurate six figure grid ref if the conditions are bad. It's definately no substitute for a compass and map - but it's a sensible addition.

I shall be carrying out a review of the Etrex Summit fairly soon - but for now I'll concentrate on the mount.

Click the pictures for a larger view...

Garmin make a specific mount for the Etrex range - so don't bother making your own - it's simply not worth it.
The mounting kit has a part number of: 010-10267-00 and is labelled as a handlebar mount.
It is aimed at cyclists - but is perfectly adaptable for standard renthals. Not tried it with Fatbars - but I suspect it will work anyway as there is plenty of adjustment. It comes with two rubber strips that grip the bars as the hinged mounting is clipped around the bar - then tightened with the small bolt (visible at the base of the mount)
I only used one piece of rubber - (the thicker piece) and it is rock solid. It may be an idea to loosen it off slightly so it could move in the event of a crash. Says it's okay upto diameters of one inch (2.5 cm)

Conclusion
It's probably a bit of an extravagance to buy a GPS specifically for your bike. At about £200 - the Summit is around the middle of the Etrex Range - having a compass (that works even when you're not moving), a Barometer and Altimeter. The Yellow Etrex is the cheapest but only really acts as GPS and cannot store or display as many routes than the Summit.

The mount appears to work well - and so fa I haven't suffered from vibration related GPS failure - but heed the warning above. You'd be pissed if you lost it and didn't realise until a mile or so later!

The top Etrex handhelds - the Vista for example, can display a map downloaded from a PC. Sounds great but is limited to the major roads etc - it won't be any use for hiking or green laning.

There are very few sites where you can download routes anyway. If you have one however, it's £14 well spent because it means that it can be used to record routes dead easily as well as checking your position against your map.
If more people did routes as GPS downloadables - it would make for a good day out following other peoples routes -and save the tedious stopping and starting to check the map.

I shall be doing a long route around Derbyshire's White Peak soon. I have been working on it already - and I will provide a complete Garmin compatible downloadable route on this site.

The GPS has a waterproof back panel that covers the battery compartment. The kit has a different backplate that you must replace your current one with - it has the mounting lugs and locking tab built on already. It means that you can remove the GPS from your bike and take it with you - or remove it and put it in you bag if you plan on riding hard and risk coming off.
It has a rubber O-Ring seal that enures it can still be used in a downpour. The GPS can in fact stand immersion in water - so no worries there.

This is where I mounted mine- so it's an angle I can see it. It could get knocked off if the bike goes over - and I may consider rigging something off the central tensioning bar - where the green cover is - but I only really have it mounted on the bars when I'm out logging routes or following downloaded routes from other people.

___ A Word of Warning
Several sites I have checked out do say that the vibration caused on bumpy tracks can cause the garmin unit to switch off.
Also - a secondary method of attachment - such as a lanyard is recommended - should it shake loose you'll not lose your expensive GPS!