My GPS III came with a basic mounting bracket (standard I believe) that allows you to till the angle of view. - it looks like Genghis' one has one as well.
I found on all piper yokes the cheapest and easiest way to mount is with two strips of Velcro (with a combo of hooks and eyes) to tie round the yoke. I have never had a problem attaching to the yoke with this method and never had the signal drop out - even when once the panel mounted GPS did!!!
The problem I have is trying to describe this in detail without a picture. So here goes.
On most yokes there is a horizontal bit and two vertical sections at the ends. If you position the mounting bracket (with the GPS III attached) on top and in the middle of the horizontal bit of the yoke you could then see that if you rap a strip of Velcro around the each end of bracket and the yoke you can tie the GPS in place. Using a combo of hooks and eyes on each end this is what I successfully do. [if someone can draw and post a picture of what I'm describing that would be great]. Total cost of mounting kit has come to approx £2.
If my feeble description still hasn't helped, PM me and I'll take a picture and email you.