Great idea. I like that pocket. It really solves a nagging problem.
I do like BB351 does in constructing the skins. I use grosgrain on the seam though and leave it on the outside also like BB351 does.
One thing I've been doing with my skins though. I use one long skin from end to end instead of 2 skins that meet in the middle. I was having trouble with pulling the skins towards the middle. I always ended up with a balloon there and trying to get the air out was a hassle since the silnyl is air tight. I solved that by just rolling and pulling the skins the whole length of the tarp from one end. No balloon any more. Also, I don't taper the skins for the tarp since there was no more bulge in the middle.