The channel on the ends only has to be big enough for the cord. I just eyeballed it.
Through the channels I used some bulky 3mm nylon cord from REI that I had lying around.
If anything, your pitch might be a little too steep. The rule of thumb seems to be 30-35 degrees and you are looking closer to 45 degrees. Those anchor points are very close together. This might be part of the reason you feel a bit cocooned, but some shoulder squeeze and cocooning is normal. At the steep pitch you currently have a Ridgeline won't do a thing for you, but as your anchor points get father apart I think a Ridgeline is a must. It just makes setup so much easier and longer spans more usable.
I also use ripstop from Joanne's and, Hancocks. Maybe you got a bad few yards? I've got the equivalent of a few months in one of my DIY hammocks with no signs of stress.