You might squeeze in a small anoloko 42 or maybe the small Possum from Alpkit.
Otherwise it might be saddle pack time.
Alternatively, a small backpack / camelbak type thing might do it. A bit MTB but might be better on the terrain. I also have one of these you could borrow
Anton_D
dommyracer
I bought an alpkit Glider frame bag (half frame for use with bottles in cages) which is decent for the little money
You're welcome to borrow it if Dov's doesn't fit your bike (his was too big for my bike so I ended up buying one)