Different Size couplers available. Holes are perfet D shape will tightly fit to the motor.
For 6 mm shaft Gear motor you get screw attachment sysem with coupler as motors comes with hole on sahft. But got 8 mm shaft big gear motor, you need to attached the coupler by araldite gum for permanent.
Choose the shaft hole size and flange coupler dia from above options.
Shaft Size of Motor
Flange coupler Dia