When it comes to that sort of thing, good and cheap don't always mix. Especially if you're planning to use it to hold up several hundreds of pounds of bike!
Thules do a kit, which will fit perfectly and be rock solid. Not cheap though.
Alternatively, go the root a few of us on here have, and fit a detachable towbar and a thule towbar mounted rack. Probably no more or less costly, but keeps the biks out of the wind and thus improves economy, and also enables you to tow a trailer (which I find very handy at times)