Here is a great value for a quality air disc. These air disc diffusers are perfect DIY bottom drain retro fits. Air disc measures 9".
1. Threaded Connector 3/4 inch NPT (Male) Inlet2. Glass Filled Polypropylene Diffuser Body3. Premium Quality Membrance EDPM4. Glass Filled Polypropylene Retainer Ring5. Check Valve Feature6. Air Inlet Orifice Assembly7. Concentric Membrance Perforations8. Hex Headed Shank