I have explored many of the commercial & do it yourself sites. Too much $$$. The cheapest & easiest way I found was to buy metal roof sheething (20" wide by 50' per roll). It is light, easy to carry out of the store & manageable. I cut pieces (3' long), slightly bent it to form a gutter type drain & fastened it between the roof beams at a 45 degree angle. I then fastend a second piece overlapping from underneath to the end (easy to work with). They all drain into a $5 gutter that drains to the outside. I used 4 rolls @ $30 per roll & 2 gutters (10' each) @ $5 per gutter & galvanized
1 1/4" wood screws. My area was about 16 x 16. The width between each beam was about 15".
Total cost was approximately $135.