I'm copying this over from another thread:
I enjoy building all kinds of stuff, so when I started looking at cartop racks, I decided to build one. Now I'm almost ready to mount my 2x4 cartop bike rack. The rail is a 2x4x54" with a 7 1/2" crosspiece attached to one end with a lap joint. I then cut a channel in from the end opposite the crosspiece up 22", 3/4" deep, and 1 1/2" wide with a dato blade, smoothed the channel with a straight router bit, the half-rounded the edges with a router. I've mounted a Saris fork mount to the crosspiece after sanding and a double coat of flat black exterior paint. 3" stainless u-bolts will mount it to my factory luggage rack. I'll only be carrying one bike (mine), so I won't overload the factory rack.
I'll get some pics up as soon as I can. Has anyone else here built a rack before, cartop, hitch mount, etc?