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...
Step 2: Housing Mount
-ANOTHER UPDATE- I like the idea of having a rubber gasket between the PVC and the housing. I am testing screwing in the bolt all the way and putting a rubber hose gasket between the housing and the plug. Seems to work ok and provides good friction for adjusting left and right.
Step 3: The Light Mount
Note - I chose a slip plug here because I do a lot of shore diving so I can easily pull the light off for easy entry and exit but under water there is enough friction to allow me to swivel the light - A threaded plug here might make sense for you.
Step 4: Assemble the Rest
Step 5: Drain Hole!
Also I got paranoid and added a VERY SMALL air hole next to where I screwed in the bolt for the camera mount to allow air/water flow just in case. Check your arm to make sure there aren't any chambers that I missed...