just an idea, I'm a gps wannabe,

first put gps into waterproof plastic case, I think "waterproof" might be the same kind of designation used for hand-held VHF radios. In otherwords resistant to waterdamage caused by a hose of some flow rate held at some distance, "submersible" is resistant to water damage held submerged for 30 minutes, clever ,kind of like watches that are "water resistant to 50meters" or just "water resistant" I don't think anything electronic is waterproof for long.

So here's the idea. cut a wedge shaped chunk of minicell that will be the base for the gps, wedge shaped to angle it towards you. Base carved to angle of deck. Glue velcro to back of waterproof case other velcro to top of minicell. Depending on where your deck bungies are located you can have the deck bungies hold onto slots cut into the minicell. If the base needs to be stronger (so as to not be distorted by bungies)you could glue a plastic (polyethylene) stiffener onto the bottom and glue another strip of minicell or neoprene to keep it from sliding around. The lanyard on the pouch would still be looped onto bungie. I find an under deck net bag is good for tossing stuff like this into when things need to be stashed safely and quickly.