After a long search for a fin i didnt find the right dimensions and a good one that fits the box i have and i decided to creat one and after testing i ended up with this modelThe Fin hight is 13 cm and the base lenght is 12 cm Total lenght is 17.5cmbox plugs info in picturethere the side model and the midle one, miror both to get the other side of them F3D FILE AND STL included

i will also put the link to the box that fits with the fcs fins i created they work really great and fit perfectly

https://www.thingiverse.com/thing:2505709

i used silk pla and petg and both worked i sold some to the locals and they liked the silky ones , well enjoy this guys and happy surfing