Perhaps a cheaper, but still bespoke solution, is possible.

Hillsalive http://www.hillsalive.co.uk/page11.shtml
(Rob Hill - well known in MSCC circles) makes a pair nylon material (cordura-type, in a range of colours) fitted bags that fit behind the seats rather like the French luggage advocated by H3 but a fraction of the cost.

And the suitcase used by Steve S is the Samsonite American Tourister, which has a weather sealing o-ring - but personally I think the large sized one he offers is too big. I have a medium size one (auspiciously, bought some years ago without this use in mind!) which can be strapped to the luggage rack without casuing as much interference with the rear view. I plan to get a local sailmaker to make me a cover in black waterproof "duck" material.

I do like that French luggage, though...

Nigel