This speaker enclosure is for two 13mm x 18mm speakers (example Mouser part # - 665-AS01808AOWPR). I designed this around the tender interior for the Bachmann On30 Modern 4-4-0. It will slip into the round speaker opening and below the circuit board. The circuit board will need to be unscrewed to socket the enclosure into the speaker hole. Depending on how generous the circuit board wire lengths are, some may need extending to fit over or around the enclosure.
The speakers can be wired in series or parallel - whichever best suits your sound decoder's requirements for your chosen speaker.
Fine detail plastic can be brittle if mishandled. Take care when cleaning this item. Was the parts with warm water, mild dish soap and a soft brush (like a child's soft toothbrush). Rinse soapy water thoroughly and allow the pieces to dry.
For assembly, the best method is to secure one speaker with two tiny dots of ACC on diagonal corners. Let that set, then secure the second in the same fashion. If you try to wire first then glue, the strong magnets will cause them to repel or flip. Once secure, apply Aleene's (or similar) glue with a toothpick to complete the edge seal. Just enough to fill the tiny gap. Using a pin vise, drill a small hole at the back of the cover shell, then feed your wires and solder as needed. Secure the shell with dots of ACC, then seal the edge with Aleene's.
This will fit other Bachmann round speaker openings in On30 tenders but may require some modification of posts in the tender or minor filing of the enclosure due to screw placement or size. If reshaping is needed file or sand; don't cut.