I used a large rolltop dry bag (Sealline 55L, IIRC) as my checked bag when I went to Iceland, the blue one strapped to the top. One pannier had clothes, that was my carry on, the rest of the panniers just got packed into the blue bag. It'll roll down quite small, or give you extra storage space. I used mine on the road for my sleeping bag, water bladder and other beverages.

Only thing it didn't contain was the sleeping pad, that was wrapped around my bike frame on the way there and left in a freebie pile of camping goods at the hostel when I left.