I use a Karitek rack on my pop top to carry our open canoe.
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I already had the Karitek rack that I used on roof bars on a previous car. To get it to work on the VW I had to buy roof rails (glued and bolted to the pop top) angle plates and new flat cross bars. All that was over £300!
The whole lot apart from the roof rails can unbolt and be removed when not required. The roof rails are lower than the solar cable housing, so make the van no higher.
I would recommend the Karitek if you can afford it. I can easily load a heavy (40kg) Old Town Discovery canoe on my own.