Each of those L brackets has a long side and short side. Use the long sides to make them reach where you need them.
Also, there are supposed to be anti crush collars in the holes of the plastic. This aligns the holes in the plastic but prevents clamping pressure from smashing it/extruding it thinner. You only tighten the bolts to compress the collars. The collars and washers hold the plastic bits in proper position and alignment.