This is an extra-large pointer ("chicken head") style knob. I was afraid that mating the 3D-printed shape directly to a quarter-inch shaft wouldn't be strong enough, so this knob is an overlay for an existing commercial knob.
The original knob was one of these: https://www.sparkfun.com/products/9998
The "Basic knob cutout.stl" file is a plain 3D model of the bulk shape of the original knob, with no detailed features or shaft hole. The "Basic knob cutout with hole.stl" file is the same, but also includes a cylindrical protrusion from where the set screw is located on the original knob. In other words, it is the shape of the cutout needed on the bottom of the jumbo knob in order to fit over the original knob and allow access to the set screw. These files are provided in case you want to design a new knob; the cutout is already done in the main STL file, "Jumbo knob.stl".
The original knob fits very snugly in the cutout. Some force is required to insert it. You may find it helpful to warm up the printed part with hot water before inserting the knob.
This knob was created for the console of Organ Donor, http://organdonorproject.org
The cutout for the original knob requires support material, but the hole for access to the set screw does not. Use a brim to help anchor the edges of the round skirt.
The skirt bent upwards when the knob was removed from the heated plate. I soaked the knob in hot water to allow the skirt to relax into its flat as-printed shape.
The support material in the cutout has to be completely removed. Any remaining support material could prevent the original knob from going all the way into the cutout.