I was looking for a low cost way to collect rain runoff from under my deck while creating a nice, dry storage area.
You can get something professionally done that looks super nice, but this is a pretty concealed spot, so I was not concerned as much about looks.
I ended up installing corrugated metal panels (the cheapest panels I could find - plastic would work too). While some instructions online suggest a specific slant to ensure runoff, I found is was easiest to simply mount the panels at three points: to the deck, a single 2x4 in the middle and two stacked 2x4s at the bottom. I installed the panels in an overlapping fashion to prevent leaks.
The most difficult part about this project was cutting the metal around the 4x4 deck legs while still ensuring runoff. In addition, I had to modify a hole around my dryer vent to prevent obstruction.
A final note - we have lots of wasps and other critters here, so I thought ahead and installed some wire window screen in places where there were gaps (along the sides and where the dryer vent was cut out). Our spacing between deck boards is pretty small, so the only thing that could really make it under there now are insects.
A final tip - I have small kids and was concerned about sharp edges, so I installed some pipe insulation on the ends and ground/sanded any rough spots or corners elsewhere.
I've had this up for several months and it works well for dry storage and drains right into my vegetables and flower bed!