What sealing type for cable/tube enter casing for outdoor use?
The general type of thing you are looking for is a Cable Gland. There are different types and you might want to look at IP ratings to get a feel for how waterproof and dustproof you need to be.  E.g. IP68 is waterproof and dustproof for outdoor use and short term immersion, IP54 for outdoors but protected from the worst of the rain or in a dry climate.

You might also consider: 1) Having the cable come in through the bottom, so water running down the cable runs away from the feedthrough not towards it. 2) using a connector rather than putting a bundle of cables through a gland - this is a little more expensive but usually works out easier if it is to be regularly removed and reconnected.