> And it appears that the cargo sits directly on the roof of the vehicle

Let me talk about that. I had a roof-top carrier that did indeed sit directly on the roof of my car. I recommend against that. It collects water when it rains, which seeps up into the carrier, and it abrades the roof something awful.

My understanding of a roof rack is that it us supported on structural components of the chassis and does not rest on the roof. There may be rails, or the rack may attach too and rest on the gutters, but the load is not supported on the roof itself. The metal (or whatever car roofs are made of these days) is not intended to support heavy loads. (I used my carrier for sleeping bags (how I found out water seeps up into the carrier) and for dirty laundry (yeah, I found out about seepage that way, too, but I didn't sleep in my dirty laundry).)

> Are there some decent, sturdy racks out there, maybe with their own metal
> mesh rack affixed to the frame that would hold a useful amount of weight?

That's what I would look for - a rack or support system that attaches to the frame and which has a basket for the load that keeps the load on the structural parts of the vehicle. "Useful amount of weight" is in the eye of the beholder. I have seen several cars along the side of the road on the way to Burning Man which had tipped over and spilled the contents of their roof rack into the bar ditch. I suspect a combination of speed and swerving to miss a cow or rabbit. Heavy loads on the roof definitely cause stability problems. You may not want a useful amount of weight on top of some vehicles that are reported to be tippy. Be very careful.