On chromatic accordions, we simplify the diminished chord by the 7th of the bass preceding the desired one.
Example, for C dim, we play the bass of C and the 7th of F
for G# dim we play the bass of G# and the 7th of C#
for memory, order of the basses starting from C and going up, C-GL-RE-LA-MI-SI-FA#-DO#-GL#...
attention for the purists, this version to play the diminished chords and a simplified version