Not sure if this is helpful anymore since the post is a few month old, but…
I have a Boofsquire rack that I brought back from the US, had to sell my 370Z when I left and bought a Renault 265 here, so I don't have any use for it now (would be interested in selling...)
As per the above the rack is fairly basic and I did fear it falling off, but it was fine and New York roads are quite are strong test. I couple of thing I noticed which I think would apply to the boofsquire or a homemade version:
Due to the slope and shape (slightly narrowing toward the back) of the boot the rack would slide backwards slightly, placing it part on the spoiler helped
I got some helicopter tape to protect the paintwork, otherwise the rack movement would damage it, this also helped a bit with the sliding backwards
The cross bracing bar seemed very close to the rear screen, never seemed to make contact even with 2 mountain bikes on the rack, but think it could damage the glass if it did