Dear Ernst,
I am astonished by the magnificent project in your workshop!
What could be the military use of a heavy MK108 firing downwards without any way to aim this weapon? If you agree that the answer is 'none', you have the answer for your question
As you have written, such canons can be found in a gondola beneath the fuselage or wings perhaps, that would make sense. For such details you better look into one of the many hundreds books written about the Bf109.
Good luck!