I've made a simple device for mounting extra equipment using the stem.
I did this so I can keep using the complete handlebar for riding.
PVC Tubing (Diameter slightly bigger than stem)
round bar (length according to the amount of extra stuff you want to use)
flat piece of iron (to connect round bar with pvc piece)
Rivets (you can use small screws)
Step 1: main piece
This is the main piece,
sized by trial, filing away small bits to get a good fit
make it long enough so it covers the whole stem
the front bit is wedged between the 2 top bolts of the stem (see final picture)