Thanks for the link! I ended up using a piece of plastic on the bottom of the bag to keep it from bending down. I wouldn't be worried about it except that it's going on a full suspension and I don't want the tire hitting the bag. I haven't decided if I'm going to sew the plastic in or not.

I built my own seat bag: http://bikepathwarrior.blogspot.com/2010/03/how-to-build-seat-bag.html

Here's what it came out like:

Thanks! It was less than $20 for everything. It was about $15 for one yard of the fabric and a few dollars more for the velcro and zipper, plus shipping. I bought the thread at Joann fabrics for about $3. Turns out you can even buy similar fabric at joann fabrics, you just have to ask for nylon or ripstop, so you could get everything there and save on the shipping. I'm sure the joann fabrics fabric isn't exactly the same as the white widow cordura but it's similar and would probably work.

It was time consuming, mostly because I had never sewed before and I had no idea what I was doing so I had to figure out how to use a sewing machine and everything else. It took around 12 hours of somewhat focused work this weekend. If I did it again I could easily do it in half that since I know what I'm doing now. Someone who has made other projects like this and has all the right tools could do it in two or three hours I think. Some of my time included buying a silver marker because I couldn't see black on black, buying more thread because I bought the wrong kind the first time, etc. The usual the first time you do a project like this. I'm building another one for my commuter soon so I'll let you know if it goes faster next time.

Thanks for the responses. I sewed a bag and wrote up how it went here: http://www.bikepacking.net/forum/index.php/topic,667.0.html Moving on to a seat bag next.

I built a frame bag this weekend. I posted step by step instructions on how to do it at my blog:
http://bikepathwarrior.blogspot.com/2010/02/how-to-build-bicycle-frame-bag.html

Here are the results (ignore the excess velcro, I'll trim that once I'm sure it's right):

I'm building a frame bag out of 400 weight cordura. Should I use a straight stitch or zigzag stitch to join the panels together? If zig zag, is wider better? This is my first sewing project. Thanks!