Here is a short video of my bike rack : http://www.youtube.com/watch?v=3TSMnRLg9KY
I broke my rule of going the free route and purchased a real fork mount for $20 at my local bike store a few months back. You can see in the above photo how it sits now. Definitely a lot more sturdier. I'd recommend a fork mount or build something very similar to it!
So technically, to sum it all up (instead of going through the steps ahead) a 2x4 of your length, a fork mount (or something similarly built), and a few tools is all you'll need. Hope this has inspired you for ideas!
Ever since I got a new mountain bike not too long ago, I've really been wanting a bike rack for my SUV. I had one of those 'hang on the back door' kind of bike racks back then and I wasn't a big fan of it. I was always afraid and looking back to see if it was still there. Don't have a hitch so I couldn't get the bike rack hitches, obviously ;). I'm a real big fan of the roof racks. Gives the vehicle a great look in my opinion and it's just cool to see someone driving with a bike on their roof.
With those 2 facts being my only upside, I had to think of the down sides. Clearance issues, driving habits, the hassle of putting the bike up 6ft on the roof each and every time...just didn't seem too worth it.
So the only way I could be worry free, was by having it in the trunk! Without just tossing it back there of course ;)
*Note : I own a Ford Explorer. The width of where the wood fit was 52inches.