The other weekend I enjoyed waking up in the rain under my DIY silnylon tarp. I'd say it worked almost perfectly. This video shows how the rain beads and rolls off the tarp nicely. There's only one tiny spot the size of a dime that a droplet was bleeding through... that's it!
It cost me $8 for the silicon and paint thinner, and about $20 for the nylon. It's about 12 x 10 feet in size, and weighs 563 grams. It would likely cost me $150 or more to buy a tarp that size I figure.
It does look a little unprofessional. For some reason, the silicon did leave white blotches in areas, I don't know what caused that, and also a patch where it stuck together while drying.
Here's some pics of the tarp with close ups of my tie outs.
Corner pullouts with jumbo sized hair elastics for tensioners to hold the doors closed.