Luggage wanted/design query

Having taken the commuter topbox off for a major fettle of the GS, I am
reluctant to put the ugly thing back on now the GS is in its finery, so,
what I am looking for is a nice-looking piece of luggage with the
following criteria:

1. easy fixing/removal from rack with single catch (preferably lockable)
2. about the size of an attache case and capable of taking (say) an A4
box file and overtrousies, phone, sandwiches etc
3. straps so it can double as a back pack (carrying a helmet around
shops etc is bad enough)
4. 'office friendly' (ie it looks like it belongs there rather than on a
bike or half way up a mountain!)
5. don't mind whether hard or soft - probably soft but with a backing to
provide some rigidity?
6. nominally watertight
7. under 20 quids

Any ideas? [1]

TIA

Chris D
[1] I thought of using the tank bag but it is a bit on the small side
and doesn't fit the rack too well <G> - I don't like keep slapping the
magnets on and off my shiny tank either!