We do some racks for Yackers (cause their money is green too, not because I think grown men in those silly things makes sense) but mostly do racks for bikes. A sturdy set of rails from one of the leading brands is money well spent. I like Thule, but Yakima, and others are good too.
Don't worry about the roof slope, just spread the bars as far as possible so that they are leveled, or the front one is slightly lower. I like ratcheting straps better than the carriers, but you have to make sure you don't crush the yak. Last two things, no reason a small car can't carry a yak on the roof. You can put one or two rails on it, then a hitch receiver with a "tuning fork" attachment, similar to what you see on some pick ups, and have it supported further back. AND.... Loading and unloading a large heavy yak on the roof solo, sounds like a good idea, but for all but the young and strong, isn't.