I needed a place to hang my headphones so I went and printed one.

This attaches to an Ikea Lack shelf (which is 50mm thick). The ABS will bend a little under the weight from the headphones. Or at least this one did (printed with 2 walls and 0.3 infill).

Version 4 has the bracket bend inwards slightly. This should reduce some of the sagging of the bracket under load.

I'd recommend using a little bit of double-sided tape (or any temporary adhesive that won't damage the shelf) to attach the hook to the shelf.