Select a lightweight adhesive. I used tacky glue.

Glue the battery into the compartment

Glue the battery controller board to the battery and position so that the ribbon cable is aligned with the connector. I used a large rubber band (or sponge) between the power controller board and the mother board to temporarily hold position while the glue dried.

Glue the extra mass to the battery

Reconnect the battery cable to the mother board. Tilt the black lever up, slide the cable in, and tilt down firmly.

If you have succeeded, you'll be able to charge the battery while the glue dries.