You could try something like the Altura Arran 16. £26 for a single 16 liter one from Evans (UK). Simple, but does the job. Take two for additional capacity.
Altura dryline has the added benefit of being waterproof. £80 for a pair (so 64 liters capacity) from Wiggle. I have both, and they're both ok. The drylines are a bit sturdier though.