Suggest printing with TPU. Cover is optional if you desire some additional weather protection for the batteries. E6000 adheres to TPU quite nicely. Add some tiny magnets to the flap. There is a 2 and a 3 battery design. Original design is on Tinkercad if you want to resize for other batteries types.
https://www.tinkercad.com/things/dqtqpagv8N2-bls5-battery-box
https://www.tinkercad.com/things/5l6MAQxkT9O-bls5-battery-box-cover
The author marked this model as their own original creation.