My kid needed a shelf to put some stuff for his bed on, so I made this modular shelf for him. Will fit any 3/4in(10mm) tall by 6in(153mm) deep shelf/plank.

I included a shelf you can print yourself that was designed to be about 12" long when everything was assembled (print shelf piece twice). You can also print the Shelf Extension piece and extend the length of the shelf to as long as you want (will need to print enough Shelf Middle Supports as well).

I also included a shelf hole guide for you so you don't have to measure everything yourself.

Printing:

Only note here is that you should leave the shelf parts on the printer bed until the printer bed has cooled down, then remove it. I used PETG as I had some spare and ruined a shelf piece because I removed it right after it was done printing and it warped beyond saving.
Parts:

Shelf Ends: 100mm tall, 163mm long, 30mm wide.
Shelf Middle Support Flush: 94.25mm tall, 163mm long, 30mm wide.
Shelf Middle Support Locking: 100mm tall, 163mm long, 30mm wide.
Shelf: 10mm tall, 153mm long, 188mm wide.
Shelf Extension: 10mm tall, 153mm long, 188mm wide.
Shelf Hole Guide: 99.53mm tall, 1mm tall, 191.75mm wide
Installation:

Install one end first, then the middle support you choose, then the other end.