Not much to it. I wanted a place to put my drinks while sitting in a hot tub, and I didn't want the drink containers touching the surrounding warm water and heating up.

In other words, this cup holder provides separation between the drink container and the water it is floating in.

Prusa i3 MK2S

Doesn't Matter

No

0.2mm layer height

Solid

Use solid infill. The design is already hollow. Using more material for the outer walls will likely decrease the chance that your floating cup holder develops a leak in the future.

Also, notice that the top of the part is flat. In other words, the top part of the "doughnut" is flat. This is the surface that should lay flat on the print bed.