don't know of a boot bag, but oxted do bulkhead bags here
I use one of these http://www.force4.co.uk/7310/Gul-Waterproof-Holdall.html in the boot of the CSR, so you might want to check measurements if you are not an SV, its squishy, totally waterproof and is a brilliant fit leaving a little space at either end for bits and pieces, in my case tyre pump at one end and first aid kit at other end, I would imagine the problem with "fitted" luggage in the boot would be getting it in and out past an FIA bar then past the boot cover even with it undone it gets in the way, also remember the floor of the boot is larger than the opening at the top. The Gul bag is bendy and pliable and as water and, to an extent airtight, you can use it as a compression bag
Tim