To support my BB I used a 20mm diameter curtain pole.
I therefore made an adapter to the BB support for the original pole.
I then designed a cover cap to mask the base with M4 screw, where the post will fit.
_____ and
I revisited the perch (to contain the head of the M5 screw inside), and the two side fences (with M5 hole). Also in the left fence, by inserting an M3 nut at the end of the print, I can hook the European Robin in 3D!