@
antimonysarah I don't imagine the following is what you want to hear, but even so, I believe this is the most practical solution.
I bought this bag:
https://www.ebay.com/itm/ROSWHEEL-8L-10L-Bicycle-Cycling-Waterproof-Pannier-Bag-Saddle-Rear-Seat-Carrier/253309867024?hash=item3afa71d410:m:mZIKaLOuYNSE1oHqqe7RP6A
You saw mine on LOL. This is a cheap knockoff of much better bags. Disregard the word "waterproof" except in, shall we say, an aspirational sense
. I think the designer misunderstood how the straps are supposed to work, so using this particular bag is a bit challenging. A better one would be better. But it holds a lot of stuff, and when properly mounted and well packed, does not sag.
I have this exact bag and, though I have many misgivings about the design, I have no regret about buying it. It is cheaper than a rack or a support for the bag you have, and will serve you better, so in that sense I recommend it. I have not tried the more expensive versions, but I suspect it would be worth spending more.