This is a replacement for the fischertechnik construction toy part no. 37232 - Verschlussriegel.
I also added a re-design with hexagonal 8mm head and a nut wrench for the redesigned part.
By experience I would recommend to raise the print temperature to the upper range given for your filamen.
Also, a very slightly increased flow rate (1% or so) might help to increase layer adhesion.
My standard settings for the PETG used are 220°C and 103% filament flow, which resulted in parts that broke at the transition between head and shaft.
I ended up with 245°C (exceeding the proposed range of 210-230°C) and 104% flow rate, resulting in parts that actually lasted.