In reply to flaneur:
Easy, lowe alpine crag attack 42
Used it for this purpose and was perfect. Didnt cram it full, because had hold luggage for metalwork anyway, but had a motorcycle helmet and soft odds and sods in it. Just took back piece out and when compressed empty space with straps, looks innocuous and fit in gauge (albeit 25cm, but can underfill and squish)
Is also a brilliant crag pack - comfy, appropriate size (feels a lot more than 3L smaller than bergaus arete 45, don't know which is off) simple, sturdy, neutral colours available, cheaper than alternatives.
Has taken a bit of abuse and looks the same, half the people I climb with have ended up with them, can't really go wrong trying for money, often in sale.