Unscrew the front casing only of the BT wall box, plug this purpose made box into it and re-screw this box into the remainder of the original box (no new holes). Then use as normal.
This post shows the final unit as it would look, because I removed the whole BT box from the wall not just the filter device on the front.
So, you can practise on / see how this back and front box fit together before moving the front section to your BT box.
BT sent this wall box to me to improve broadband speed and it remained until fibre was installed. So it is not needed anymore.
Taped together for storage purposes while not screwed on.