I literally took the big plug, pulled it out, and cut it off of the cable/wires going in. Never had a single problem with water. (comming from someone who lives in FL, who constantly has over 6" of standing water in their driveway/roads)

You can use a screwdriver, then bore it out with a drill bit, just be careful.

And it looks like 4ga wire in that pic, but it might be 5....
I ran 4ga through that hole, powering a 2000watt amp, to a 15, and the 4ga was plenty.