Final thought is it is probably not too difficult tomake a replacement using black sugru
take a trace of the triangle shape into paper
Squidge the sugru onto a flat non stick surface to say a couple of mm thick. (Maybe lightly oiled baking paper ?)
Take the traced paper shape, place on top of the sugru splat and then cut around with a knife
Wait for it to set. Voila , black triangle rubber thingy
Might need to then glue a tiny bit of elastic to the battery and to the new flap or something Like that anyways
Never tried this but don't see why it won't work. Outlay a few quid only plus time ?