I have several 4.4 cu ft mini frigs that each hold 3 2 1/2 gallon kegs. I have drilled a hole in the side of each for the temp probe and for the CO2 tank for my keezer.
The best way to drill a hole is from the inside.
Any coils on the sides are next to the exterior wall for heat dissipation.
Drill a small hole just penetrating the inner wall. Use a step drill.
Stick a bamboo skewer in the hole and clear out the insulation.
Once you are able to feel the exterior wall you will be able to tell if there are any cooling lines at that spot.
You can feel it as you fish the bamboo skewer around.
Once you are sure its clear, finish drilling all the way through the exterior wall.
To clean up the hole or make it bigger, use a step drill, it makes the cleanest hole.
A regular twist drill will just make a really ugly hole.