To waterproof the wires, think about getting a role of 3M mastic tape. From below, wrap the wires several times to make a plug that won't pass through the hole. Pull the wires up through the hole until the plug is exposed. Then, using a bit more mastic tape around the wires from the top (make sure it touches the stuff coming from the bottom) so that the wires don't fall back down.
You have to stretch the tape out so that it activates and "melts" together, but the tape won't damage the sheathing on the wires. My father-in-law uses this stuff at Boeing whenever they need to water- and weather-proof an electrical connection outdoors.
Wow, that installation job is first-rate!