The clips on that were what I was looking for on my boat tail. I made some myself by bending metal for the top though. That bag looks much better than using a box on top of the car. My superior packing abilities make such extra space useless I took my box off since I never used it, but I see a lot of people that could benefit from a bag like that.
Those clips would be really easy to duplicate with simple hand tools. A little high density foam tape padding on the inner surface of them would make for a nice tight rattle free fit.
I like the idea of making an easily detachable boat tail though. Using ABS or other similar material, a shorter one could be made to fold up and store inside the car while driving around town. Go out on the highway for any significant distance and it could be clipped on in just a minute or two.
Snax, that's exactly what I am working on. It's basically a small wooden frame that extends from the back, and with cordage to keep it tight on there. Right now I'm planning on going cheap and using a clear trash bag on the top trapezoid, and cardboard on the rest. If that all works than I'll have a removable boat tail that only cost me $3 in hardware (the rest was "found objects")! I'll make a post of it once I start assembling the cardboard in the next few days.