You are so stupid for wanting to make your own gear!
Why on earth would you want to make your own equipment. You are so dumb......you need to just buy name brand gear like everyone else. You should only do things that other people think are cool, never act on your own desires when it comes to your personal gear.
Man I hope the mountainbuzz community does not represent the real river community, many of you come across as real assholes.....
OK now that I got that out of the way.....I make my own gear all the time, not necessarily to save money, but because I enjoy it.
You could either make your own stuff sack and put a grommet in the bottom, or maybe put a grommet in the bottom of a commercial stuff sack.
Don't forget to put some sort of buoyancy in the bag, like a cut disc of foam pad or something.
As far as the rope goes, you want it to float, be strong enough for your application, and be thick enough to hang onto. Too thin and it hurts your hand....
Hope that helps.