The pencil holder shown in the picture has the supports tilted outwards. I don"t think it"s the best option, not because of the stability but because it takes up more space and doesn"t have an "ergonomic" look, so I modified the project and arranged the vertical supports. I have attached the project file in FreeCAD and STL so that it can be printed. I only printed the walls without a core, it is very easy and it achieves its purpose. I used PETG, for PLA which is more fragile I recommend printing with a core to support the rigid walls so that they do not crack under the loads.

    You can modify the project as needed; I bought 10 pens annoyed that I never have one next to me and I found that I needed to keep them together in a jar and I came up with this idea that I like. It"s something else!