O River,
LL Bean and campmor.com sell foam blocks to slip onto gunnels, and then set the padded boat up onto a car top. Straps come with it. I'm not fond of these rigs as they often seem to slip around and reposition themselves where THEY want to be. And, frankly, they never want to be in a good spot.
Best home made contraptions I know of say either Thule or Yakima on them. Plan on several hundred $ to get the attaching towers (the part that clamps to the vehicle). the cross bars, and saddles of some kind to hold the boat. Oh, and straps.
Frankly, home made racks run the gamut from as good as store-bought, down to a burlap bag and clothesline. If you drive a pickup, PVC or wooden racks can be quite serviceable. Fewer options on a sedan or coupe.