The tricky bit is the left wall because that's where all the cables are. But you now know exactly where to press :) See Step 3 Picture 3

(Picture 1) Using a flat-head screwdriver, push in Catch A and poke it upwards. The plastic holder will not completely be free, but you want it to move slightly upwards.

(Picture 2) Same thing, using a flat-head screwdriver, push in Catch B and poke it upwards. Use your free hand to gently pull the plastic holder (so that it does not fall back in place inside Catch A).

If you've done it correctly, you can just jiggle out that stupid plastic holder from here.