The ring contains small air pockets that make one side slightly buoyant. Five embedded US pennies on the opposite side add just enough weight to ensure the ring sinks straight down while remaining vertical.

Printing Tip:
Pause the print at the appropriate layer and insert five US pennies to achieve the proper balance.

For it to not just sink but sink vertically, it must weigh exactly 50 grams in total, including the coins. Assume that five coins weigh 12.5g.

Feel free to copy or modify the source to customize the model for other coin sizes:
https://cad.onshape.com/documents/ed60f07c313672725ddfa494/w/cab31260417268bce42037fb/e/541b63d85f9aee1682658fb2