Hello Gelbster,
the good idea of the Pedros / Ernie design is that you fix the head of the bolt with the bar itself. There is not much / no room to access it, if you want to mount it above.
Shure, you can also mount it below and use the parts you've mentioned. I personally would always prefer the above solution.
In general i think the products are overpriced, because they are not too complicate to fabricate.
There is also a triangular solution. Don't know how good product quality is:
PORSCHE BOXSTER 986 ULTRA RACING 4-point Rear Lower Bar UR-RL4-1105
Because it seems to be jammed on the connection points, i think it's not as stable as the Pedros / Ernie design.
Regards, Markus