Basic width breadboard (a-j) cannot house more that 10 pin wider test boards.

E.g ESP8266 CH340G NodeMcu V3 (31mm) shield leaves no free pins for dupont wires to connect.

This extension allows to use 12-15pin wide shields leaving 4 to 1 pin connections free on the breadboard side. Extension board allows one dupont pin connection per line.

The first time of dupont pin used, place wire(s) on the side hole(s) and then press the shield pin(s) in to breadboard side hole(s). This will mold the dupont pin with small dent and after that dented pins can be connected even after shield is in it's place.

(Though the dupont pin gets a minor dent the breadboard accepts those as well)

The small holes are challenging to print are the true hole size depends on many variables. The hole size can be tuned on the slicer. E.g Cura has 'Hole horizontal Expansion'. My setup requires '0.2mm' setting to get a snug fitting with connector pins (0.67mm) of the test board.

I included short quickly printing Test part for your convenience to set up your hole expansion parameter.