A cheap and practical solution is to use electrical "junction boxes" (French: boites de dérivation).
They can be fairly cheap, are widely available, and were tested for industrial deployment.
For instance, you can find 85x85mm IP66 cases for 3.5€ on Conrad, but really you can just go to the nearest hardware shop and they'll have them as well.
Of course, if you really want them to be waterproof you're going to need cable seals. Interestingly enough, you can wedge those 3dBi plastic antenna into a seal from the inside of the box in such a way it form a water-tight seal. Of course it's not completely weather proof (indoor antenna...) but it's a nice touch and it makes the case look like a giant phone.
EDIT: just like that: