I have several bags to suit differing situations. Amongst these are two from Kata, a shoulder bag and a backpack. The quality of materials and construction is superb. Ffordes have a used DC445 in stock, at £25. Whilst slightly larger than your requirements, it's always better to have room for phone, wallet etc rather than cramming stuff in a bag. If I need more room I use a Thinktank Retrospective 10, probably beyond your budget used, but the RR of camera bags. Here's a link to the Kata http://www.ffordes.com/product/14041613092631 You can also check out other bags here http://www.cambags.com/