Anything can be useful, at least that's how my daughter thinks. So she's got a little mess on her desk.

To try to help with that I've created a simple shelf over the Lack table.

Those brackets do have aligning edges, and countersunk screw holes (m4)

The shelf itself is an old Ikea shelf cut to dimensions (55x25cm) and it has an edge with 19mm height.

It is stable, it works. Highly recommend :P