Reptibreeze cages are hard to make drainage for... With my medium cage I put it on a table with the front edge hanging off about an inch. I then propped the back up with wooden wedge so all the water ran to the front. I then put a catch pan on the floor under it where the water would collect.
With my custom cage I am building a stand with a square hole in the center that the cage will sit in, Under the cage will be a drain pan with a hole in the center drilled so water will drain into a bucket hidden in the cabinet where my supplies will be stored.
I dont think there are any "Proper" drainage systems on the market. Just gotta be creative with what you have available to you.