The Slant Top Cap works with Fountain and The Whiz. The top and bottom are both 14mm Outer Diameter

Like the Turbo Top, you can use either side.  One side offers a straightforward quartz plug, while the alternate side not only seals the quartz but also provides an adjustable airflow feature. By rotating it, you can fine-tune the airflow restriction, effectively covering the intake tube. It is fun to do, and in testing people enjoyed being able to play with the restriction.  Personally, I dont think it is necessary to restrict the flow.  I tend to favor the simplicity of the basic plug side, and the slanted top design adds an appealing aesthetic touch.