I've had this c/v Eagle Creek fanny pack kicking around for a while, and came up with a pretty simple re-purposing as a saddle bag. It's a good size for my needs - I could easily add a small camera in addition to the pile of stuff shown, and I left the straps long enough to cinch a wind shell.
The process was pretty simple: a) I stitched on 2 leather patches to retain the straps and a third for a blinky; b) I cut off the belt strap and stitched up the edges so they wouldn't unravel; c) I replaced the zipper pulls with cord to keep things quiet. The straps are MKS toe straps.
If I find that it bounces around too much for my liking, I can easily put a loop of shock cord around the seatpost, but it doesn't seem like it will be necessary.