what battery for trunk?

ALL batteries vent, even the AGM types like the Optima as well as all sealed lead acids; any overcharging of an AGM will activate the vent under pressure.. To do it the right way, put it in a box that is fairly well sealed, and provide a PVC tube to the outside, usually below the trunk floor.

Any other venting into the truck area, or regular airflow will help, and may make the tube vent not necessary. The objective is to remove or highly dilute any hydrogen gas.

Werner Franz, one of the last survivors of the Hindenburg airship crash of 1937, dies at 92