I use a medium sized rack bag to carry the tent. If it is wet, we separate the dry inner from the wet fly and pack the fly in a plastic rubbish bag to stop other items getting wet. I used to carry a sleeping mat in the bag until it bought a more compact mat. I also pack a thermorest chair on long tours and a spare tyre in the rack bag. The rack bag attaches very easily to the clips on the panniers and there is no problem of rubbing holes in the bag as I had happen when I used a dry bag strapped along the length of the pannier rack.