something similar using 3x rebel boards and took what seemed like pretty basic precautions:
- Fixtures themselves are under the eaves of the roof, thus can't be rained on under normal conditions
- Enclosed the LED's using a sample jar with a glass lid
- Used RTV silicone on the wiring penetration to seal it
- Ran a half-bead of RTV silicon along the backside of the sample jar lid just to ensure it didn't fall off
The fixtures won't get rained on under normal circumstances and are reasonably splash-resistant. Any moisture that does penetrate can diffuse out through the unsealed half of the lid. Insects can't get into the fixture.
I did nothing for the solder joints - those will likely be the major weakness of the fixtures.
My 4-fixture installation has been going strong on a >75% duty cycle (I typically turn them off during the day in the summer) for more than 18 months now. They survived the brutally hot summer of 2011 and don't seem to mind the occasionally-horizontal rain from the spring storms around here. Dallas is kind of humid, but has nothing on locations closer to the gulf where relative humidity generally stays 80% or more ... not sure if more moisture ingress protection would be called for in such locations.