I recently did this to clean water out. One of the tabs on mine (visible with the headlight installed) was filled with glue, but it broke easily with a careful application of a small screwdriver, and didn't impede reinstallation of the cover.

I put a small bit of RTV around the seal between the body and the clear cover before installation to help it seal, then let it sit overnight with the back cover off to cure. It's been water-free since.