I sew my own, too. It's pretty easy if you have a sewing machine. I do something like this (hopefully this is clear!)
1. Cut out a rectangle that's as long as I want and twice as wide (I usually don't measure, just eyeball it. I think it's somewhere around 18" long and 14" wide.)
2. Cut out two strips to be the ties, about 10" long and 1.5" wide.
3. On the big rectangle, I fold over one of the short edges, which will be the finished edge on the top of the tail bag.
4. I take the two strips I cut out earlier and fold them over twice along the long edges so it's three layers of fabric and is 10" long and 1/2" wide then I sew down the length.
5. Then I fold the big rectangle in half and kind of eyeball where to pin the end of each tie so that they'll be roughly across from each other once it's sewn. It doesn't really matter if they're exactly across from each other. Pin them so about an inch overlaps with the bag, and the other 9" stick up away from the finished edge.
6. I sew the ties onto the finished edge where I pinned them, taking care to make sure they're sewn on REALLY well.
7. Then I fold the rectangle in half along the long side (so now it's 18" long and 7" wide) Then sew along the unfinished short edge and long edge.
8. It's currently inside out, so flip it "inside in" and it's done!
I've done it with stretchy fabric or with regular cotton, either works fine, though stretchy fabric can be more of a pain to work with!