I used the diygearsupply.com patern and 7 yards of ripstop silnylon from the $1/yd clearance table at my local Ben Franklin Crafts store. I'm very happy with the way it turned out. Total cost was less than $15 including webbing, shock cords and a spool of the expensive thread. I rough cut the panels and french seamed them together, and then my mom took an interest and took over, marking out the cat cuts and sewing the rolled hem. Its too bad that she let me cut the cats, because I messed up and nicked a piece I didn't mean to touch. She patched over and under it, and the patch has held up as well as the rest of the tarp.
It took me a whole evening to mark the panels, another to cut and fire seal them, and most of a Saturday afternoon to sew the french seam. I did a really good job on the french seam. Everything is heat sealed and double stitched. I have used a contrasting color thread on all of my hammock related projects so far, because it's a but easier to inspect for damage or failure.
I used nylon webbing for the tie outs, and each of the bottom tie outs has a shock cord loop for strain relief. Do you use shock cord at the ridge line tie outs also?
Here it is with the cat cuts marked out in chalk and the panels pinned together so that we could cut both at once. We basically just guessed at the arc of the cat cuts, using a piece of cord as a rough guide. They don't have to be perfect.
My wife and I took it on a camping trip a couple of weeks ago, but we used it as a foyer for our tent since we only set up hammocks in the day time for napping.
The ground was way too rough to bring the tent any further under the tarp, and those were the last trees until the road, so we couldn't set the tarp up any closer to the tent. Either way, that didn't matter, because it kept the front of our tent perfectly dry. I never saw a single drop on the under side of the tarp, even though it rained heavily a couple of times. I put our fire bucket under the corner that is behind the tree in that photo, and it collected well over a gallon of water the first night, and about a half a gallon the rest of the week. The faux cat cuts worked perfectly, as we never heard a sound from the tarp.