20 litre containers can be used for other things also. I use one cut in half as shown to catch the oil under the car when doing an oil change and put...
Step 7: Attach the sides and back
You can then attach the sides as shown with the n shape at the bottom and the 2 L shapes on in the centre and on on top. make sure you orientate the sides so the hole are around the right way and the with white strip on the front. It is simply glued and screwed on. The back can be marked out and cut to size and glued and stapled on with a large staple gun.
It is a good idea to not move the shelf for a few hours until the glue dries as it will be quite heavy and easy to break.