I ended up using 6" (blue) reusable rubber twist ties ($3 @ Home Depot) to fasten carabiners to the metal car frame -- one on each side of the trunk (hatch?).
Since the twist ties hold their shape, preshaping them like a hook helped with threading it through the holes in the metal frame. Threading it took a few minutes since it's not easy to reach into the metal structure behind the cut-out in the plastic molding in the C-max (The photo above is looking up into the cut-out from the floor of the trunk).
The 6" ties just make it through the two holes in the metal support and around the carabiner. If the ties came in 8" or 10" it would probably be better/stronger... but the next size seems to be 12", which for me, left too much twist-tie left over (I didn't want to double it since I don't want it accidentally interfering with the seatbelt which also passes through the metal support).
Since I used the twist tie to fasten a carabiner, I can fairly easily hook/unhook the cargo organizer using the caribiners. So far, these rubber twist-ties are doing the job well. I'll see if it holds up over time, or if heavy groceries and taking sharp turns ends up pulling the carabiner out of the twist tie grip.
Edited by Red, 09 September 2015 - 08:18 AM.