Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:5904980.
My daughter and I made this simple reading guide together. She struggles a bit with reading and we decided to make this (without searching Thingiverse first, I know there are many others out there). I wanted to do this as a learning process to learn everything from measuring with a ruler to printing the final product.
First, we made a prototype with paper to test it out. Then she took some measurements and made a sketch before 3D modeling it in Tinkercad. We used Prusaslicer to prepare it for printing on the CR-10. It took a few iterations before landing on the final design, testing everything from different lengths to thicknesses. We ended up with two widths for the "reading window" to accommodate different line heights.
Later I remodeled it in Fusion 360 and added some user parameters to easily change the total width and the width of the opening. All in all a fun project to make a useful little thing.