What makes you think it;s the heater box itself? are you suffering a coolant leak inside the car or is it just not cooling? There are a bunch of vacuum pods that will open flaps to blend doors that if not working will cause poor cooling
I have had a further look today and got the two main halves apart. One of the pair of flaps seem to be very loose and I can't really see how they should be fixed to the linkage to avoid the floppiness.
There is also a video here: https://photos.app.goo.gl/3dH7EEf6NmQ37YMK6