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

Eross: "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."