8/18/11 UPDATE:

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.

Step 1: Parts and Tools

I used a drill and a wrench.

The parts I used were all found in the garage.
-51inch 2X4 (trunk width measured to 52inches)
- A type of 90degree bracket (No clue as to what specific type it was to be honest. Been in the garage for years and I didn't even know.)
-3 washers
-2 nuts
-2 o-rings
-2 screws
-1 adjustable turner (not in this picture, sorry. If you click ahead you'll see it at the end of the bolt and realize what I'm talking about)
-1 bolt

*Note: This was per side.

Step 2: Position Where You Want the Fork Holders to Be

Once I gathered all the parts I needed, I took my bike and positioned the fork how I wanted it to be on the wood. I was going to line it up simply parallel to the edges of the wood, but as I moved it side to side, simulating a turn if you will, I felt that the parallel position might weaken the 90degree mount from the wood. So i positioned the fork diagonally and I like it. Felt a bit sturdier as I moved it side to side slightly.

After marking where I wanted my 90degree brackets, I drilled in the screws to hold them in place.

Step 3: Place the Fork in the Holders

Once everything was set in place, I put the fork in between the o-rings. These moved around quite a bit for me. I knew ahead of time though because the bolt's diameter was way smaller than the bracket's diameter. Had to work with it though.

*Note: Those screws you see on top on the brackets serve no purpose. They were just one there when I found them and just kept them on.

Step 4: And Just to Make It More Secure

I drilled out two small holes then wrenched in those bolts you see there at the bottom of the wood. As you can see in the picture, they are there so I could secure the bike frame with a tie down just to be sure.

I took a test drive with it, and it held up very well for me. Checked up on it afterward and it lay just how I left it.

You know, this may not be top of the line, but I didn't spend any money making this, and best of all I felt great after it was complete!

Enjoy!