My daughter wanted a seat to carry a doll on her bike, which makes sense as I transport her on seat on my bike. Looking at Thingiverse, I quickly found a seat that seemed to fit the bill. I had to cut off the top of the seat, as the Flashforge Adventurer I’m using has a limited print volume, and then I noticed the fixation system would require a long piece and would generally not fit my daughter’s bike.
So I redesigned the fixation system. Basically instead of a long 3D printed piece, I used a 10 mm wood dowel and I designed a new piece to fix the bit of wood to the stem of the saddle. I cannot say the design is super elegant, but it requires less complex pieces, and it works. To use it, you still need to print the original seat, and then the two fixation pieces, one that connects to the seat with two M4 screws, and one that connects to the stem with two pieces of tie-wrap. You can cut the 10 mm wooden dowel to whatever length feels appropriate. The third hole in the seat side part can take a wood screw that holds the dowel in place.
You can access the designs on TinkerCad: