Personally I would not use a LED. They tend to be inconsistent in operation and their peak sensitivity if not always matched to human vision. Which is probably what you prefer.
The tept4400 ambient light sensor is spectrum matched to human vision.
Plus is comes in a "LED" type package so it is waterproof. Stick if out of a correctly sized hole and it would be OK. You could always epoxy it or silicon seal on the reverse.