I made my first frame bag out of Tyvek from USPS. I REALLY dig it and it seems to be holding up well. If you're going to do I'd suggest doubling up your tyvek. Either use spray adhesive to bond to pieces together or use a heavyweight fusible interfacing inbetween each layer. Don't let your iron touch the tyvek though. Keep the temp low and keep the iron moving as tyvek will warp/melt easily.
Also, there seems to be different grades of Tyvek. I happened to find like 40 ft. of tyvek on the side of the road one day while on a ride. It says "stuccowrap" or something similar. The weave is different than USPS envelopes and it seems to be slightly more robust.
You can made a bag out of (3) priority envelopes. Check in the trash at the post office for discarded ones before wasting a new one.
Here are a couple photos: