I travelled around Europe for 6 months last year with a Piaggio scooter on the back of my westy with no problems.
There is a guy in the uk called Dave Cooper
(see
http://www.davecooper.co.uk/ ) who makes them pretty cheap. You need the bike first, as dave makes them exactly to fit your bike length and tyre width. FYI I have the bike for sale 800 quid if you want it. (cost me 1600)
The biggest problem is is the weight of the bike. VW says that you should not put more than 100kg on the back (including the rack). I think it is called 'tongue weight'. It is in your owners manual.
We had to get a 50cc Piagio, and have it de-restricted so it was about 80cc, as there were no 100 cc bikes that were less than 100kg.
We loved it, and had no problems whatsoever.
I would highly recommend it. Give Dave Cooper a call.