i did a month tour with a large dry bag lashed to my seat with pedal straps. constantly had to adjust to keep the bag off my legs. also squeezed the contents of my dry bag with pedal straps to keep the bag in a fixed position.
i thought about getting a seat post rack. my frame is small, and the seat post rack would not give me much space below the seat. plus, i think seat post racks are slow, or slow looking.
after thinking about this problem for a week or two, i decided to keep trying. i have some generic cargo nets that are made out of bungee cord material. they will squish my bag like the pedal straps
so i have some cheap poly rope or small gauge cord that i'm making rope net out of. it will give me the flexibility to move my bag without squishing it.