I just finished our utility bike's pannier bag. I copied Xtracycle's design as much as I could (their pictures were tough to steal ideas from). It's made of Cordura I bought from Rochford Supply. I sewed the entire thing on my industrial machine, as my home machine did NOT like the cordura, and of course didn't like the webbing. Plus, the industrial machine stitches with super strong nylon thread, which is more durable. You can't rip it with your hands. I made a few mistakes on the first attempt, but nothing bad enough to require a re-make. I used snaps from Very Baby to attach it to the frame, and webbing from EMS/REI for all of the straps. A salvaged 1 1/2" car seat belt was used for the middle adjustable strap. Any more spec questions, just ask. We have yet to use it, so I don't know how many grocery bags it will hold, but we'll see. I will probably use our grommet kit to put some grommets in the bottom, as rainwater will just sit in the bottom if we don't take the time to empty it.
Tray and I are considering opening an Etsy shop with our bike seats, accessories for family bikes, and probably custom panniers.