Can I Plug the Tee when the Cell is Removed?
You may need to plug the open port in the tee when you are cleaning or changing the cell. Any standard 1.5″ threaded male PVC will work.

For example, you can try this one from Amazon.
If you plan to plug the tee permanently, you must wrap the threads of the cell with Teflon tape. If it is temporary, that is not necessary, but it will prevent dripping or leaking.

Links to Amazon are affiliate links