Wow - you lot are fantastic! I didn't want to reply and quote one answer in particular as they're all good, but I do particularly like Thomas' "several paving slabs forming a cube" idea for the box.

And John's mention of a cyclonic filter is interesting; not sure I'd need the magnetic particle removal bit of it, but it seems no more expensive than the non-magnetic versions. I'd have to check what the min flow rate it could handle. I'll try a pair of tights for now though (over the end of the intake pipe I mean, not on me!).

Anyway, I'm armed with some good ideas here. Thanks all (but do keep them coming if you have an idea not covered here yet!)