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interesting idea on the coroplast. I've always had the tent in mine, so the shape of the rolled up tent in its bag has kept the shape well enough. Generally I've always kept stuff in the rackpack that I only take out at end of day, so it has always kept its shape well enough due to teh stuff in it. Ive sometimes stuck a watermelon or whatever stuff bought off a roadside vendor in Latin America, so I've always found it handy to have spare room in it, same idea for end of day groceries before heading to a campground.
My tent and other dense stuff goes in panniers. I only want the lighter stuff on top. That said, sometimes the 1.5 liter bottle of wine goes on top, that is not very light.
I first used a pizza box that did not have any pizza grease for a stiffener, used that for a few trips. Then when I knew that it was something I would keep using, switched to Coroplast.
Some of my tours, by end of tour I had so little food left that I no longer need a pack on top in back, on two trips I was down to only a tent pole bag at end of trip. But those trips I usually used a dry bag that I could fold up and put inside a pannier at the end, the Rack Pack stiffener in the folding part would make that difficult. And I do not want to discard the Coroplast, so if I might want to fold up my bag on top later, I use cardboard that I do not mind discarding later. I have since cut different tent poles that fold up short enough to fit in a pannier.
My hiking shoes are usually in the bag on top in back, they are not a convenient shape for packing. But otherwise, my bag on top is almost always just used for food. Some trips in colder weather where I have some light density clothing, that will go in the bag on top too. And it is convenient to have a bag that can take a lot of volume. On my Maritimes trip, I saw a giant plastic box filled with really delicious looking croissants that was on the discounted day old bakery shelf. That box fit in my bag on top nicely for the several days it took to consume the croissants.