Two ideas I've got (and I need to be quick, gotta go in five minutes) --

1) Get a rack that has adjustable width for the arms. I had to do this to get one that'll carry my wife's Electra Townie, and it'll carry my Cannondale Rush at the same time. Just barely, though -- and I don't know if it'll work with your RumbleFish the same way.

2) Get a "top tube adapter" or whatever they want to call it. It's a bar that attaches to the head tube/stem and seatpost to make a ladies' frame bike fit on a carrier. They will work with mountain bikes, too.
http://www.prolineracks.com/bike-bar-adapters.html

Ah, Yakima has one:
http://yakima.com/shop/bike/bike-accessories/tubetop

And Thule, too:
http://www.thule.com/en-US/US/Produc...-Frame-Adapter