Today Mirapro opened the hatches giving access to the top of the Bottom Filter. Then I did a first inspection of the BF:

•  I idenfitided two exactly opposing places where to put the ballast masses. See the pictures. At the +Y side the place already hosts ballast masses whereas the one at the -Y side is free. The reflection of the free hole is seen on an inspoection mirror.
• I confirmed the security structure locking screws for the BF are accesible. See the pictures.

Observations:

• Because the hole at the -Y side is free, we also need to take into account the mass of the screw.
• At the +Y side the existing ballast is held with a very long screw, which will likely need to replace by a longer one. We need to assess its length and the mass of the replacement. It's BUMAX/SDC M6.