The new Kuat Mini Skinny roof-top cargo basket gives you somewhere to stash your extra gear without taking up the space needed to also transport your bike. Measuring just shy of 24″ wide, you could likely put a roof-mounted bike tray on either side of it. It comes pre-assembled, but is also available with an extender to increase its capacity for wagons and vans with longer roofs. Or go minimalist and use the integrated 9mm QR fork mount for that road bike…
Full measurements are 60.5″L x 23.875″W x 6.375″H. Add the extender to gain 14″. The front has a built-in fairing to improve aerodynamics and (try to) send love bugs over your cargo instead of smashing into the front of it. They say it retains 50% of the usable cross bar space and comes with locking roof bar mounts. Retail is $295 for the main unit, and $89 for the extender. Pre-orders open now, shipping in September 2017. Capacity is 160lb.