What is the best way to mount an Xtracycle to the roof of your car?
Xtracycle has a few suggestions particularly a Thule or RockyMounts Tandem rack.
We have a Mazda CX-9 with factory cross bars and no hitch.
Our bike is a converted Bianchi which looks similar to this one. It's around 50+lbs.
7ft of Cet-Bak
Would installing a hitch come in handy for other things? If not, I'm a fan of the Rocky Mounts tandem roof tray. It can be a little cumbersome to load/unload by yourself with a tallish SUV, but pretty easy with an extra set of hands.
A regular advertiser in Recumbent/Tandem Magazine is the Draftmaster.com ..
.. their Topper rack, perhaps? Tandem Topper One-person Loading Roof Top Racks