I know not all have access to a lathe but, I machine a receiver groove for an o ring in the face of -12 coupler or bulkhead fitting. Then use the appropriate size O-ring from the 8 dollar Harbor Freight o-ring kit. Seals against outside of pan. No leaks ever...You may be able to buy like this but, the only ones I see have the o-ring recessed into the threads where it wouldn't seem to be as effective for this.