After deciding that all the tandem-specific racks are far too expensive I decided to see if I could modify an existing one to fit a tandem. It seems to me that the basic mechanism that holds the front of the bike would work for a tandem just as it would a regular bike, so if I could just extend the part that holds the rear wheel, it would work just fine.
I decided to use the Swagman rooftop carrier: http://www.swagmanbikeracks.com/shop...cat=275&page=1
Based on its reviews, it works just as well as others and is much less expensive. The plus was that if I wasn't able to modify it to make it work, I wouldn't be out as much money.
I ordered an 8' piece of 3"x1" aluminum channel iron to move the rear wheel holder out. I'll let pictures do the talking.
As it turns out the aluminum bends quite a bit at the forward roof rail. I'm going to strengthen it over the front rail with some more aluminum I ordered today. But it looks good. It is quite stable (aside from the bending of the aluminum rail, which will be fixed), and I can open the rear hatch!
With the extra aluminum, so far I'm out about $120. I'll update once I modify it with the strengthened piece.