DIY/MYOG bikepacking seat pack sewing guides (and extra guides for a basic zipped frame bag and stem bags) : r/bikepacking
Bikepacking Hacks: 3 DIY Seat Packs - YouTube
Creative Gear Ideas for Bikepacking on a Budget - Exploring Wild
Made a Roll Top Saddle Bag for tools, light jacket, snacks etc. Rolled down it's around 0.8L and full stuffed and buckled is around 2L. Utilized a voile strap from saddle rail