My shower is over my bathtub. I like to plug the drain whilst showering to retain the water to simultaneously bathe my feet. This gadget saves me fumbling for the old plug. Just leave it in the bath drain outlet receptacle and open/close it with a super easy two finger 45° turn of the rotary vane valve knob.
Fully clockwise is valve open. Fully anti clockwise is closed.

This fits both my bathtub or my kitchen sink. Both have 42mm outlets. I expect it can be scaled up or down a little to fit different size drain outlets. It’s intended to have small clearance to be a snug drop fit into the receptacle. No need to push it in as with a conventional tapered rubber plug. If it’s tight into the receptacle, it will distort the vane assembly and prevent proper sealing and vane valve rotation.

EDIT: I just added a smaller version of the plug which fits my washbasin outlet which is 35mm. So the 3 files which are prefixed with S- are the smaller set matching the 35mm plug hole. I had to redesign them in order to maintain minimum wall thicknesses etc. Just scaling down the original 42mm plug didn’t really work as everything got too thin to properly print and function as intended

I’ve tried both TPU flexible and PLA rigid. Both work but the TPU is best being far more suitable and durable and it conforms nicely to seal better against the outlet. The water pressure helps push the surround thin flange down to the outlet surround face. It also helps the valve vanes to mate closely when closed. I’ve not tried any other materials but I expect ABS or Nylon might also work well?

The water retention seal when closed is good enough for my purposes. I suppose it would very slowly leak and empty the bath or sink (even when closed) if left filled for hours.
I have included 2 option versions of the rotating control vane, both with a knurled knob for easy gripping with wet/soapy fingers. One has an extended length protruding knob (high profile, optimal for easy finger gripping). The other is low profile making the knob almost flush with the drain top surface (probably best for kitchen sink use)