So in mid-August, we traded in our 1995-ish Ford Ranger (rated 18 mpg) for a 2009 Honda Fit (rated 29 mpg). It made me sad, but the transmission was likely to go out in the next year, and there was no way we'd get $4500 for it any other way. The only worry with it was whether or not we'd be able to transport one or both of our bikes in the back hatch. Amazingly, the answer is yes. It's a tight fit, but we can even put a third passenger in the car with both bikes, which is more than we could say for the Ranger. I also hadn't noticed this post at Fit Freak before I started construction, but they're both pretty similar to what I ended up with.
Trek 7.2FX (mine)
Specialized Globe Carmel (hers)
Start with a 1x6 measuring 50 inches in length. Check that this fits snugly behind the rear wheel wells if you orient the board vertically. Cut a 4"x4" triangle off two corners to accommodate the rear wheel wells and let you place the board flat across the cargo area.
Prepare some firring strips to reinforce the 1x6, so that the bikes' weight doesn't cause it to sag. I attached four strips roughly 9.5" in length around 12" from each end. Your own lengths and positions will vary according to where your bikes end up.
Remove the front wheel from both bikes, and see how you can arrange them upright on the floor so that their leading surfaces line up, and that they take up as little room as possible from side to side. In our case, this put the front fork of the 7.2FX about 2-3" behind the front fork of the Globe Carmel. This is when I also noticed that in this position, the Carmel's rear wheel stuck out quite a bit further than the 7.2FX. And it was quite a bit taller -- all this indicated that the Carmel was definitely going to have the tightest fit.
Next, do a test fit of both bikes into the cargo area. This is a job best done with two people, but it's possible to do solo if necessary. Depending on where you position the front seats, you may be able to butt the rear wheel of the bikes against the back surface of the driver's seat, or like us, you may have to position one rear wheel between the driver's seat and the door, and the other rear wheel between the front seats. Place each bike onto their front fork block, make sure you can close all the doors and the hatchback without bumping into anything, and then mark the final positions of each fork block.
Our fork blocks (one Yakima, one Delta) required drilling 1/4" holes. Since the only 1/4" screws I had on hand were too short to fit through both the blocks and the full depth of the 1x6, I drilled 1/2" countersinks on the bottom of the board. Go ahead and attach the firring strips somewhere near the center of the board's span, or near where the fork block attach.
As can be seen below, the Carmel makes for an especially tight fit, but there's no interference or contact anywhere.
All other photos from the installation are at Flickr.