A somewhat simple stopper (bung, cork, plug, stopple &c.) for bottles and the like. Set it to the size you need.

This has two extra options apart from the standard circular plug:

• With a notch that fits a string or rubber band to hold the plug in place
• Scaling the plug in one direction to get an elliptical shape

The filleting at the edge between flange and stopper proper makes the definition of the diameter a bit ambiguous. Maybe you want to do a test print of just the bit above and below the fillet and adjust the size.