A GPS bicycle mount can be a real issue. It must:
• be bulletproof-solid
• be rattle-free
• allow access to all GPS controls

RAM makes great molded cradles for Magellan eXplorist and Delorme PN-20 handheld GPS, but there exist no standard parts for fixing these to either handlebar or stem.

However, a pair of conduit clamps (shown), found in the electrical department of any big home improvement store, work great, and they are trivially cheap. Mounting the GPS to the stem is superior, because it points forward and there's room for 2 clamps.

Conduit clamps come in a variety of sizes. A 1-1/2" clamp will fit a stem of 32 - 36mm diameter. A 3/4" clamp will fit a 1-in diameter stem or handlebar.

Generally, use 5mm or 6mm capscrews and nuts to fix the cradle to the clamps and to pinch the clamps around your stem. Use stainless screws if possible, and slip rubber pads under the metal clamps to prevent scratching your stem.