I do body harnesses for BDSM, cosplay and animal roleplay purposes. For the central join point I used metal rings. But they are expensive and because they're round, they wrinkle the straps.
So I created new plastic "rings": Square for the bottom of harness and pentagonal for the top part. They are strong enough and look better.
The components are intended for 30 mm straps. The buckles and other plastic components are store bought. It is possible to 3D print them, but they're readily available and pretty cheap, so there is no need to print them. If you do want to print them, there are models on Thingiverse as well.
The following parts are available:
- Hollow version ("rings" as on photographs)
harness_5.stl - pentagonal top part
harness_4.stl - square bottom part
- Full version (solid construction and holes for straps)
harness_5F.stl - pentagonal top part
harness_4F.stl - square bottom part with four holes (for additional strap going between the legs)
harness_3F.stl - square bottom part with only three holes
I recommend to print with 100 % infill for strength, but you may lower the infill if the harness is mainly for show.