Here's a Paint picture, hope it makes it clearer:
Two L brackets support the trap at the wall/trap junction. A single metal strip runs all the way around the trap, secured to the wall at the rear side. If the trap have a frame or some cardboard corners where the metal strip meets the trap, it can be taughtned very hard. If the metal strip is totally taught, the trap will hang flush with the ceiling.
Alternatively, two metal strips (or strings or similar) could be used instead. If so, the L brackets could be left out. Though, given that these where hard enough to mount as is, even with the helping hands from a friend, I did appreciate the L brackets to help support the weight when mounting it!
The ironic thing is that these 4" traps (with some scrap cuts thrown in on the rear to fill it out) are the thinnest absorbing patches in the room. Yet, they where the hardest to mount! There where two main reasons to do it this way. First, no holes in the ceiling. Secondly, to avoid building frames for the traps. With frames, hanging them is obviously easier. Am too lazy to do the frames! I just wrap them in cloth and seal it with duct tape. :D