i was planning something similar. Glad I saw your build before I built a full box like I was going to. Think I will follow you plan but instead of half circles, Im going to use 1/4 circles. This will allow it to butt up flat to the back wall of my brewery. I am mounting three bath fans that will vent directly out the back wall of the brewshed. I was planning on metal drip edge, but I do like the idea of j channel as well. Was planning on sloping all my channel to one corner and putting a drain to my sink.