Here's the kind of fork mount you need. Note the two holes below the axle - VERY IMPORTANT.
Get a couple hose clamps that are bigger than the circumference of your roof rack cross bars. Get stainless steel if you can find 'em.
Dig through your old parts or ask your bike shop for an old innertube. Cut out the valve stem.
Take all the supplies to one of the cross bars on your roof rack. I'm doing the front bar on my Xterra.
Choose where you want to place the fork mount. If your cross bars are bowed - place the mount in the middle so your bike won't lean to the side. Get your tube and start wrapping the cross bar.
Wrap your cross bar without overlapping the edges of the innertube so that it's wrapped as wide as your mount and forms a nice flat cushioney surface.
Put one hose clamp around your cross bar and through one of the holes in your mount.
Tighten that junk up! (not too tight yet)
Attach the second hose clamp. Stress out 'cause you're a perfectionist and the clamps aren't identically positioned. Tighten everything up now. But don't gorilla-tighten and cut through the tube.
Go ahead and stick your bike on there! Does the rear wheel hit the roof? Move your second cross bar accordingly. You can see my handy green bungee cord keeping the rear wheel in place.
Rad! You're now ready to hit the muddy trails and come back with your bike on the OUTSIDE of your car. Pro-Tip: Spin your cranks. If they'll hit your roof, use another bungee to keep them from moving.