Some self-watering insert models are out there but none was good enough for my expectations. So I created just another one. : )

The lower half can be printed with supports, but those are not absolutely necessary. No glue is required. The top and bottom half just click into each other.

I printed mine with PLA. PETG will be just as good. In my experience, watertightness can be difficult to achieve. I would recommend at least 5 layers. And still with some prints I had tiny leaks. Enough to have a damp spot underneath. Therefore I created also a coaster. You can try without it. If you place the printed pot on wooden furniture, be sure it is waterproof!

You will need a rod to connect the "floater" with the indicator. I would recommend buying 1mm carbon rods e.g. from Aliexpress.
They are light and sturdy. Cut to the needed length.

Underneath the floater, I use a styrofoam ball. That will provide enough buoyancy. Search for "EPS foam ball" or "Fishing Float Bobber Buoyancy Ball" or similar. 20mm diameter will work great with the unscaled model.

In general, these self-watering pots are usually meant for inorganic substrates for your plants but will work with soil as well. With soil, I would use a rope that hangs down into the water reservoir. This will act as a wick. The "legs" of the insert are better to be filled with LECA, gravel, or similar, in case you use regular soil. That prevents the soil from becoming too moist.
Inorganic substrates (e.g. mix of lava, zeolite, perlite) do not require this, since the plants develop different kinds of roots in these substrates and do not suffocate in soaked soil.

Pay attention so you don't drop any soil or stones into the tube. Otherwise, it might block the floater.

The planter pot is 173mm in diameter and 183mm in height.
The insert is 165.6mm x 181mm. If I see some interest in this design I will update the design for other dimensions. For now, you can scale it to fit your needs (and existing planter pots).