Any ideas how to seal this and still be able to open it again?

So, just an update as to what this is and what was happening.  This is a Motorola 4.9Ghz (Pubic safety band) microwave transciever about 3 years old.  About every 3-4 months it would fail and the case would have about a 1/2 cup of water in it.  No amount of sealing would stop the water.  Allowing the unit to dry out and it would work again.

Turns out there is a vent at the bottom with a little disk made of Tyvek or similar material covering the internal vent hole.  The Tyvek is suppose to let air in& out but prevent moisture from entering.  Turns out the little disk was held in place by a ring of adhesive.  The adhesive failed and moisture was allowed inside which condensed on the inner casing. Eventually all the adhesive let go and the disk floated out with the water from its hidden location.  You would think more throught would  go into a $7,500 piece of equipment designed to be  installed on the top of a radio tower.