But any duffle bag would work, doesn't need to be poked, and is probably cheaper than a dry bag.
Sure. I mistakenly assumed the OP wanted to wear it on the plane like a backpack since he said it feels like "orothopedic back support" and the title mentions wearing it onto the airplane. If not wanting to wear it onto the plane, absolutely a duffel bag is cheaper and more useful. Attaching a duffel bag to the backplate is still possible, just less useful as a backpack.