I built my own seat bag to go a with my homemade frame bag for some bikepacking adventures I have planned.
White Widow Spectra fabric
400 denier Cordura fabric (you don't have to use two types of fabric, I just liked the way it looked).
1" nylon webbing
5 side release buckles
1" velcro (all of the above from Seattle Fabrics)
Something to stiffen the bottom - foam? Coroplast? I haven't tried anything yet
I started out with two "Y" shaped pieces for the top and bottom. Most of the seat bags I have seen are equal height/width but I couldn't make mine very tall because the suspension moves the wheel towards the bag.
Next I sewed buckles to the ends of a piece of webbing and sewed the webbing to what will be the opening of the bag. It's just like the opening of a dry bag. I added webbing all the way around to make it easier to close and to prevent fraying.
Next I cut out a two pieces to hold the bag to the seatpost.
I trimmed these pieces down and attached straps and buckles. Then I sewed them to the sides of the bag.
Next I made the piece that attaches the bag to the underside of the seat. It's got a piece of webbing that goes to the back of the bag and another that goes around the seat rails
What the bag should look like now:
Next I made a similar piece for the bottom and sewed it on.
Next I sewed the opening next to the seatpost together.
Next I sewed velcro to the seatpost flaps. Be sure to sew it to the correct side of the flap or it won't connect.
Here's where you can finally sew the top and bottom to the sides if you didn't do this up front like I did. Final result:
I'm happy with the results but I think it could be improved. I'm not happy with the way it sags right at the seatpost. I think this could be solved by using heavier Cordura or with something stiffening it from the inside. Since this bag is made and I'm not going to tear it apart, I'm going to try adding some foam or plastic to the inside near the post and on the bottom of the bag. It's not too bad in these pics but the bag can hold a lot more and it sags with more weight. The bottom of the bag has the same problem. As you add more weight and the bag gets longer it sags more at the end and between the vertical webbing and the seatpost. I don't think it's going to fail but it doesn't look right.
I also built a front bag to hold my sleeping bag. I'll post that next.