We have a screen porch with a vaulted ceiling. The ceiling in the center is flat and then it is sloped on either side. I would like to apply tongue and grooved douglas fir beadboard planks. I will stain them first as well as put two coats of exterior poly. I am just not certain as to how to install the boards where the sloped areas meet the flat ceiling so that it looks uniform. Are there any tricks for getting a nice uniform seam between the sloped and flat areas? We also have several recessed cans that I would need to trim around. Thank you