Watertight enclosures - what production steps are required to ensure no condensation on the insides?
Use silica desiccant

This is the stuff that often comes with clothes or moisture sensitive components in a little bag, often marked "do not eat" for some unfathomable reason. It's also used in some double glazing units to scavenge any moisture that does leak in, to avoid condensation.

Silica desiccant is cheap and is available in bulk, or pre-packaged in little permeable bags. You probably want the bag, and you can stick it somewhere out of the way inside the case with a dob of glue or double sided tape.

The desiccant will take up the moisture which is inside the box when it is sealed, and also any which leaks in over time.  Eventually, it will saturate, and lose it's effectiveness, but if you use enough it will last a long time.

You can increase the life of the desiccant by minimising the moisture in the box when it is sealed. Warming the whole assembly, or blowing with warm dry air will help. Doing the final assembly in a small room with a dehumidifier running would be an option too. It will also last longer if the seal is good.  Once the desiccant is saturated, it is possible to dry it out again in an oven, and it may be helpful to do so before putting it in the box, to make sure it hasn't got damp sitting on the shelf.