if it was glued/glassed on and is now leaking, then something has cracked due to stress or movement. Generally putting sealant or even epoxy over the top of such a leak is at best a temporary fix. It would be helpful to have a picture. If you can get your hand to it, I would sand the area surrounding, then stick some wet out cloth over it. Of perhaps mat if it is a tri-corner. If you use cloth cut so that it goes 45 deg across the corner it will conform better. If you wet it out with West Six10 it will stay put and nothing will run off.

I am curious about the setup: usually a dorade box is supposed to leak out onto the deck. The pipe up stand inside the box is the only thing through the deck that needs to be waterproof. So there is something different about this setup.