Add a little extra security to your bakfiets.nl. Print with at least 40 % infill for better stability. Use a M10 and four 2,5 x 16 screws. I use the flag bracket since 3 months and it is stable so far, but I have done no serious amount of testing regarding safety and/or durability. Therefor I can not take or accept responsibility for anything a user does with the design.