why do you need something bike specific?
I travel often with either an eagle creek bag, (water resistant ) plus contractor garbage bag,
or a waterproof mountain coop (north face etc) bag,
and a elastic cargo net.
if you were putting it in place of the top case I agree to get bike specific.
you are putting the bag on the back seat in front of the top case.
kind of instead of a passenger.
so a non bike specific bag may actually be better.
as you can take it and put it sideways easier to open the top case
or on it's side so you have easy access to your side cases.
plus a soft bag you put in what you need and roll it up.
put it under the bungee.
Because a thing seems difficult for you, do not think it impossible for anyone to accomplish.