Toad Roof Rack Mounts?

Check out Yakima, they have racks that employ clips that are specific to each vehicle model.
I have had good luck buying them used on eBay and they are of good quality and stay on.
You might be able to work the clips into the rack you are building.

Rhino-Rack also makes cross bars for cars without rails. We bought a set to carry our porta-bote on the roof of the toad. Saved a few hundred dollars over going with the factory rails and cross bars.

-Michael

Yakima and other roof rack companies offer some generic kits, as well as vehicle-specific kits. The traditional approach, as I understand it, would be to attach a set of rails to the roof fore-and-aft, and then clamp legs to the rails where needed, and the crossbars or whatever to the legs. There often are also fixed leg mounts available.

Here's Yakima's intro page on custom installations.