I'm still very curious about the feasibility of this idea. My astro has "ridges" on the roof that run from front to back. They are about 1 1/2 " wide, and have about 8 1/2 " between them. There are 4 of these between the outside rails of the roof-rack. Each ridge has a metal moulding screwed down its full length. Presumably, these are intended to keep the cargo from coming into contact with the painted surface of the roof.
My idea is to create a box of some suitable wood on which to set the Airtop. I'm thinking that this box would be about the same width and length as the bottom of the Airtop, and about 2 1/2 " deep. The plan would be to cut notches into the side-to-side walls of the box to accommodate the ridges in the roof. I'm thinking that I would remove the roof rack, as well as the mouldings on the rood ridges. I would probably install the van's roof rack on the top of the Airtop. I need it there to hold my 2 X 100W solar panels. Once I'd remove the roof rack, I'd caulk all of the holes, and screw down the box directly to the roof from inside the van. Naturally I would use Sikaflex or similar to seal it.
I currently have a Fan-Tastic Vent installed at the back of my roof. I would remove it, enlarge the vent hole to serve as a hatch for passage into the Airtop, and re-install the vet on the top of the Airtop, behind the solar panels. This last part would depend on a determination of whether or not I felt that the top of the Airtop was sufficiently strong enough to stand up to this amount of alteration (including the addition of the solar panels). I should think that, overall, this approach to mounting it should make for an even stronger assembly than "stock", and I could even add a crossing piece to the middle of the box to reinforce the attachment of the bottom half of the Airtop.
This is as far as I've developed this idea. Any feedback?