Shelf mount for metal plug shelf

This constructions is an easy and cheap way, to extend the storage capacity of your shelf/cupboard.

Why this part? Additional shelves for this open cupboard/shelf (why does english not distinguish between those two parts?) do cost 25 Euro for two pieces, and the whole shelf/cupboard itself was about the same price when i purchased it. I found that quite expensive and so i searched for a cheaper alternative.

I purchased some wooden boards for 4.75 Euro each (luckily already with the correct size to fit into the shelf frame), sanded them, put some bee wax on them, sanded again and designed and printed this mounting parts (4 for each wooden board).
They do have holes for screws, but i have not drilled holes into the boards yet - they seem to lay down quite save and stable on the shelf/cupboard.