Find a strong 20-30 lb. disk magnet for about $5. Use hot glue to secure it to the inside of the cap. Some magnets come mounted in a steel casing which can be bolted to the cap for added security.
The cap is attached to the sewer pipe using PVC cement forming a permanent bond. This makes it hard to re-glue the magnet, so get it in there good the first time. The holder is then lashed to the combustion chamber in the traditional manner using pipe clamps or your imagination.