I just built one of these. $25 for the fork mount and $5 plus change for the 2X6, 1/4" bolts, 1/4" nuts and some washers. Had the store cut the2x6 down to 45" and then countersunk the nuts and washers so they wouldn't scrape the carpet. Build time was about 15 min. I can add at least two more fork mounts as needed (i.e., cyclocross season).
Note that at first I had it set up for the bike to be facing the rear but the saddle rubbed on the headliner. I switched it around so the bike is facing forward and there is pleanty of room. The bike is a size 58 road bik.