Clamp a Ikea Ivar shelf board on the side

Modify a normal ivar shelf board by cutting of the edges, like you can see on the picture. Now the board fits perfectly between the side of the shelf.

Use four metal roads to fix the board in the usual way. Use additional four metal roads in combination with the 3d printed parts to avoid that the board falls off.

For sure you can fix anything you like on top of this board. I fixed a cat staircase from petfun for my cat.