For waterproofing I normally use IP67 rated boxes. If you make any holes in one, all connectors need to be IP67 rated and used within their specifications for cables.

I normally use M20 electrical cable compression glands for the coax, but the connector needs to be fitted after putting through the gland. Always make any holes in the bottom as suggested to stop water running downwards along the cable

Two other ideas to stop condensation. One is to totally airtight seal the box and use desiccant. This absorbs moisture in the air, however if air EVER gets in again, the desiccant will soak up completely then re-release the moisture again. The trick is to replace the desiccant every time you open the box. Just keep a spare one in a hot place like on top of a water heater. Some change colour from white to blue to indicate if they are full or not. Heating them for example in an oven removes the moisture and allows them to re-absorb next time. This is a method used regularly in double glazing. The desiccant is placed in the frames of the window normally behind an aluminum edging with holes in it. The inside of the window stays condensation free until there is a break in the outer edge and the desiccant fills up.

Another method to stop condensation is to heat the inside of the box continuously. If it is a pre-amp with a voltage regulator then leave it powered up 24/7 and the wasted heat should keep any moisture airborne, although depending on the location you may want resistors or other wasteful heaters to keep the internal temperature always above ambient outside temperature. A similar thing is done in large security cameras to stop the glass from ever fogging up. Similar to a rear window heater on a car.

A lot of amateur outdoor equipment use low quality cases and waterproofing. I found a barrel type PL259 connector can be good at stopping one damaged cable from allowing water to feed down a long coax cable and damaging an entire length of cable. Where possible solder or remove all connections/connectors so even a little corrosion won't matter.

Using an IP67 case with a transparent lid is a good way to see if water is gathering inside or if condensation is forming. Having an LED on power inputs showing through a clear case is useful to show it is powered up or the heater is working. Cheap kitchen containers are not designed for outdoor use and the plastics crack under UV light.