Viscacha is 6-14 L and the smallest Restrap is 8L so too big.
I'd like to fondle the Ortliebs to see how big they are in the flesh but the 0.8 or 3L might work. They look quite convoluted though but I guess their roll-top is better than my Specialized/Topeak's zip closure.
Ortlieb ~3L or Restrap 8L.