Ortlieb has a mesh add on pocket, their add-ons typically are fitted to the ends of the bags
they are shown to expand to let you put your helmet in it.
Maybe get a mesh laundry bag, and you can secure it on the top of your rear rack load .. so things wont fall out..& Off..
My tent is put in a stuff sack not particularly water repellent, I put it up wet the next stop, if its not raining again, then is when it dries