This is an almost entirely made from scratch solution that didn't cost me anything to make. Of course it helps if you already have some tools. :)
Step 1: Parts list
a cheap handlebar mounted reflector or similar thing. Mine came with my bike as a freebie to mark the place where your headlight should go.
two machine screws with matching hex nuts or the like.
a means for cutting metal, I used a jeweler's hand saw
pliers or metal bending tool
Step 2: Measuring the metal
I've pressed the back of my gps into the tool to capture the form I need to make out of metal.