If your water is balanced you should never need to clean the cell. It should only ever be cleaned if you can see white/light-gray build up on the plates. Never use concentrated acid and always use the least aggressive means to clean it. Some people simply use the spray from a garden hose to knock off any flakes they see. If you must use acid, start with 1:1 diluted white vinegar (if you can find 6% cleaning vinegar, then that is great to use). If you feel you must use muriatic acid, then use it diluted 9:1 (water to acid). Let it soak for a few minutes or until any bubbling stops. Once it's done cleaning, rinse it with fresh water.
A rubber hand ball is usually quite good at plugging up one of the holes. Simply stand it up in a 5 gallon bucket and let the acid clean the plates. The metal coatings on the plates are very sensitive to acid etching. This is why you should be very apprehensive about cleaning it unless there is a verifiable issue that requires it. Acid cleaning the plates reduces their usable lifespan.