Truly hermetically sealed is almost impossible to do in the field. Unless immersion is expected,  when filling with resin or gel are the better options, for things like joint boxes a vented approach is often best, maybe with a swizzle of rock wool insulation in the hole to act as a deterrent to small creatures. Care is needed about the direction of the vent hole.Down into a sheltered place is best. That said,  even quite simple things like a peaked hat overhang on the support for the enclosure for both a rainshield and sun shade can make a very large improvement in how wet things get, or rather do not.

Truly hermetically sealed is almost impossible to do in the field. Unless immersion is expected,  when filling with resin or gel are the better options, for things like joint boxes a vented approach is often best, maybe with a swizzle of rock wool insulation in the hole to act as a deterrent to small creatures. Care is needed about the direction of the vent hole.Down into a sheltered place is best. That said,  even quite simple things like a peaked hat overhang on the support for the enclosure for both a rainshield and sun shade can make a very large improvement in how wet things get, or rather do not.