If you go to Sears Roebuck's Automotive section they sell roof racks and they sell separately the mounting hardware. You can probably adapt the mounting hardware to work with your home made rack.
I made a roof rack for my car to transport lumber from the lumber yard to my home. I simply use ratcheting tie down straps (two) to hold the rack to the car. I thread the straps through the open doors and then close the doors on the tightened straps. It holds fine, but looks awful. (It's only 3 miles from Lowe's to my house so I don't worry too much about it.)
Google: "DIY car roof racks", there are many examples.