Plywood screw on volume.
Our volumes are made from 18mm plywood and have a textured coating on them.
This volume has 9 T nut placements on it. 3 on each side.
the sides of the the triangle are 40cm long, it sticks out of the wall by 20cm
or 16" wide by 8" tall
You can screw these to the bigger volumes to make up even trickier problems