you can often find thule/yakima rack parts cheap on craigslist. They might not have the footings you need - but you can buy those new, or you can sometimes get them off ebay if someone's sold the car they fit. Roof racks don't cost as much that way.

I wouldn't trust doing your straps like that... and the webbing's gonna maul your paint on your spoiler. my kayak straps maul my car.

I have a hitch mount on my sooobie & have some roof rack bike trays - much more secure and I happened to find both the hitch & the hitch rack on craigslist.