on 10/02/2017, Bob Minchin supposed :
Drain holes in the lowest point are the way to go.

An outside container is very difficult to fully seal as thermal
expansion/contraction will act as a pump admitting small amounts of water
vapour laden air. The water condenses and stays inside and on the next
heating cycle the air gets pushed out but the water does not and so it slowly
builds up as the cycle repeats.
You still really need some sort of seal against driving rain. I often use a
silicone grease - Dow Corning MS4* smeared over the sealing faces and wipe
off any excess that squeezes out when you fit the cover.

* I think it has another name now (DC4?) but my toothpaste sized tube will
see me out so I've not bought any of the new name stuff.

+1

I always add a series of tiny drain holes to any fitting which is
outside in the weather. No matter what you do, water will get in, just
make sure it can get out more easily.