Lots of really cool ideas and execution! Magnets instead of drilling the roof, something I'm not sure I would ever have thought of. Also, you linked a single magnet but said you somehow bought them in pairs, here is the listing for the opposite-polarity pairs, for anyone interested:
K&J Magnetics - Products
The trunk-moved kammback is how I was thinking of doing it on my first car but never did; your execution is both proof-of-concept and encouragement for my current 05 Accord. Great idea with the magnetic fabric-mat material, very clever on the hinged attachment for the hitch-box. And I highly approve of sheet metal.
Your fabrication skills (and tool selection)(and cad/simulation knowledge) are definitely something for me to aspire towards.