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
dommyracer
Anton_D
Good point - my little childs bike actually needs one even smaller than your Alpkit one (max 400 long) so best just buy one eh... thanks for help all