I also used a horizontal sliding window vertically, the only issue I found was the frame doesn't have enough friction to keep the window open with the glass raised to allow air in, I ended up flipping the window so it opens at the top instead of the bottom, instead of cutting slots for drainage I just drilled a hole or two to let any water out, the existing slots don't make any leaks that I can see. I did add some extra channel across the top of the window for extra rain protection.

(the silver strip at the top of the window)