Craft Marker Holder

I am very new to 3D modeling and 3D Printing as a whole so bare with any settings that don't seem right. My family uses various craft markers, the challenge we faced with other holders were they were specific to a specific brand or shape. After some experimentation we figured out that 18mm squares fits all various markers we had. I then cut the middle out to just save on filament as this isnt really holding a heavy payload or gets much wear and tear.