So, what I did was put the Helix on top of the spare (in my Passat).
I bought a longer OEM "bolt" (is what VW calls it) that holds down the spare tire and then another OEM bolt with the nut. ($13 in parts from the dealer for both).
The longer "bolt" didn't come with a nut, so that's why I had to order another OEM one....to get another nut (can't buy it on it's own)
I put the spare in, used the OEM nut on the longer "bolt" to hold the spare down, then put the Helix box on top, and then used the 2nd nut to hold the Helix in place on top of that.
The spare is heavier than the box, so you'd want that weight lower in the car ideally (to keep the center of gravity lower).
And that way everything is locked down properly/not moving at all. Adding the 2nd nut (holding the spare down), stabilized the "bolt" better too.
Untitled by
Dan W, on Flickr
Untitled by
Dan W, on Flickr
I don't know how long the OEM "bolt" was on your Wagon, but the one I bought was 7.5" long (end to end) and about 3.5" taller than my OEM one
OEM on left:
Untitled by
Dan W, on Flickr
Untitled by
Dan W, on Flickr
To accomplish this:
Untitled by
Dan W, on Flickr
There's also a longer 8.5" version of the bolt, but my dealer had to get it from Canada and not as easily available as the parts above and more money. The threads were a different pitch from OEM too...so you'd have to buy two of them (to have two nuts).
What I posted above was the easier & cheaper solution, IMO
for now. 