This is a sea shell bikini top to go with the the Polaris doll (https://www.thingiverse.com/thing:3184441) and technically a remix as I use her chest to form the inside of the cups.

The top was designed to go with a mermaid remix of the Polaris doll (link TBD) I've made for my daughter.
I've used 1mm elastic thread to act as straps. One in a loop through the central ring that goes around the neck and a second piece that goes between the smaller side hoops that goes around the torso. ShellTopP1.stl is the file you want to just print the bikini top.

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How I made this.
Several steps as I'm not a native mesh modifiying person!

1. Using Meshmixer I made the back of the shells flat.
2. Using OpenSCAD I positioned the shells where I wanted them (easier to do and then repeat for the other side than in mesmixer) and export the pair as STL's into meshmixer (ShellTop.scad / ChestSection1.stl / FlatBackShell.stl).
3. Using Meshmixer I added the loops to connect the shells and provide strap tie points, merged them into one piece and then boolean removed the chest from the merged parts.
4. Finally export to STL (ShellTopP1.stl) and print!

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Notes for other BJD's

The top is designed to fit the nipple-less chest section that Polaris uses. It should be postion compatible with any of sources / remixs of Polaris that use the same chest torso. However if you want it to work with a nipple version or just alter it's position download ShellTopP1-1.mix and swap in the chest section you want to use, merge the other parts into one piece and then boolean removed the chest from the merged parts.