I beleive with very slight adaption the ones from a Bongo/several 4x4 ones that mount to the top and bottom lips of the tailgate in much the same way as a bike rack does most are removed from bongos/4x4's by their convertor -import companies as they are not desirable apparently to uk customers, so should be worth contacting any of the firms that import, convert these vehicles though as it shouldnt be very expensive and at least done in chrome as against black industrial ones otherwise available that are horrible At least using one of these or an adapted motorhome one it should be able to fold and be locked up at least help in stopping stupid idiots climbing it and using your vehicle as a climbing frame as is often also the case and should be considered in the fitting of such equipment
Ditto what Rob says when i had my White Noise i had investigated this as i had a permantly fitted roof box and we found exactly what he is saying and nowhere to get one from only real way was to get a fabricator to make one up for me which i was about to do, csot about £70 but it would of mounted correctly which many wont..