There are quite a few different ways to make a "water catcher". Several links from this google search
might be of interest to you. Ones that use boom tents or awnings seem to be the most popular.
A few pointers from using rain water collectors on OPBs...
First, make sure you have a way to divert the water, since the first few minutes will be cleaning the dirt/dust/grime off the water collection surface and isn't really suitable for putting into your tanks.
Second, make sure you've got a separate tank for collected water. That way, if the collected water gets contaminated, you haven't compromised all the water in your fresh water storage system.
Third, you really need to have several filters for the water collection system. The first can be something as simple as a coffee filter or piece of cheese cloth. This is really just to keep large piece of crud out of the tank. The second should be between the "rain water tank" and the pumps. This should be a standard household type freshwater filter with a 5 micron filter or so. The third should be between the water line and the faucet—and should be a "taste" filter that removes any chemical contaminants and such. This is similar to the NSA 100 or other household kitchen countertop filtration system.
On a lot of boats I've seen that have rain water collection systems, the rainwater collection tank has a separate deck fill from the regular freshwater tanks. The deck fill is often located to simplify collecting rain water.
Most of the rainwater collection systems were made using some-type of canvas, usually synthetic for lower maintenance.
One of the better ones I saw was a 12' x 10' boom tent setup, which had a water retention dam around the bottom end of each side about 4" high. In the center of the dam was a 1" hose fitting, each of which had a hose attached to a Y-splitter. The bottom of the y-splitter led down to where the deck fill was.