I bought two GPD-902 units and I have made two pinpointers and i have to tell you I am not very pleased with the results. The first one I made I made it like this
http://www.tomstreasures.com/probe.html
And it works sometimes and sometimes it doesn't. The best range I can get with it is around 3/4 of an inch.
I made another one with the same unit only I used a plastic electric box, cut a hole in the side for the light and switch, added a switch and a vibrator unit out of a playstation controller (you can have it beep or vibrate), used the end of the box and glued the PVC pipe into the box and used a cover with a watertight seal. This one works a little better. I think you loose some range because of the end cap on the PVC pipe, but I am not experienced enough to figure out a better way to do it. My advice is spend the $$ and buy a good one. That is what I have done.