Very true, that's why it is tricky! I plan to remove part of the spoiler to suit, such as here:
My modified tailgate spoiler to suit a bike carrier
I have a Caravelle Exec which I upgraded to from a sportline styled T5.1 Kombi. I love the new bus but as I have the seats I can’t have bikes inboard...therefore use the OEM 4 bike rack. I’m not a fan of the tailgate without a spoiler - it’s a little boxy in my opinion. I wanted a spoiler but...www.t6forum.com
My other problem is that my spoiler is attached, so I will have to slot it with an angle grinder, dremel and files...
I assume you'd need to remove the spoiler to make the necessary cuts to accommodate the bike rack? If so, THQ do a bike rack specific spoiler (although out of stock at the moment) which could be an easier option: THQ Bike Rack Spoiler
Video on it here: THQ Bike Rack Spoiler YouTube
Just thinking of your time and sanity - that looks like one fiddly DIY job!