I didn't want to pay big bucks to buy a specific arm for my camera and light so I put something together out of PVC. I am using 3/4 inch pvc because that is what I had, but I'm sure 1/2 inch would probably be just as strong and more compact.
UPDATE - I just got some great input from a friend - a mix of 1/2 and 3/4 inch pipe may provide a good opportunity to make this set-up collapsible .
Step 1: Parts List
You will need:
1 - 1.5 inch 1/4 inch course threaded bolt. (I used Aluminum to avoid deterioration)
1 - male threaded plug
2 - 90 degree elbows. 1 end female threaded, 1 end female slip
3 - couplings. 1 end male threaded, 1 end female slip
1 - 45 degree coupling. both ends female slip
1 - coupling. both ends female threaded
1 - end plug. male slip
1 - length of pvc. I used about 3 feet but you will modify to fit your needs.
1 - hose clamp (big enough to go around a good mount point on your light.)
A drill (3/16 inch drill bit)
a rotary tool to cut a slit in the light mount plug
-UPDATE- just a note, keep in mind that PVC is somewhat negatively buoyant... ok, carry on...