Obviously you can affect the cat cut when you do the rolled hem for the channel. If you inconsistently fold the fabric over, it wont maintain the same shape as the fabric edge.
I always pin my end channel - maybe 6" increments if not more - and use a small scale to measure my folds. I fold 0.5" then 1", and sew in the area where 3 layer's of fabric are, leaving roughly a 0.5" channel.
You can do the same thing with a larger channel, if that's what you want. Just make sure your folds are consistent, and try to sew along the curve as much as possible.
After playing around in the GF hammock I realized 2 things: 1) The hammock is longer than mine (mine was short because it was modified from a current rectangular cut hammock), and thus more comfortable. I think it ended up at 123" along the center. 2) The sides are flopier, and I could probably go a little deeper on the long side cat cut. Maybe 5-5.5".