Auto Refill Pot with Water as Liquid Evaporates
We often get cases where a person becomes fixated on one method, and loses sight of the big picture/overall purpose of the exercise. (This often manifests as the "XY Problem").  Your goal is not to abuse an Instant Pot; it's to humidify your home.  Step back. 

You're ready to spend money. Just buy the right thing.

You've plainly stated that you're ready to spend $30+++ on various apparatus to try to solve this.  The money you're willing to spend is much more than a proper humidifier actually costs.  So just buy one of those. If you can't bear to spend $30, then go to the Salvation Army store and spend $5. 

If you don't like misting humidifiers, there are lots of other kinds.  Misting has the advantage of a very sensible power draw, because it's not heating the water, it's letting your house's normal furnace do that more efficiently.  

This is a firestarter at so many levels

NEC 110.3(B) says you must obey equipment's labeling and instructions.  An Instant Pot is a kitchen appliance, specifically a pressure cooker with computer controls.  It does many things, but humidifying is not one of them. It is not labeled for that, which means UL didn't test for that, which means it's not a safe appliance for that purpose. 

What happens when it runs dry is a case in point. Instant Pots aren't designed to run dry where they would go into thermal runaway. There is no conceivable listed use of an Instant Pot where it would ever run dry, since operations are supposed to be on a timer or monitor. You are defeating those features to make it possible to misuse. 

Further, using it "way off-label" like this, will greatly shorten the unit's life.  Smoke the unit, and suddenly the "big savings" from avoiding a proper humidifier go up in smoke.

There are many other issues when it's used outside of a kitchen, but you say not.  

The Instant Pot already has a shutoff feature

Because the latent heat of vaporization is not a surprise. 

And the Instant Pot has a timer and a known rate of heating. 


You already know (or reasonably should know) the electric power consumption of your Instant Pot.  Yours will say, but my guess is 1500 watts, because that is the maximum allowed for a plug-in kitchen appliance under UL listing rules. 
The latent heat of vaporization of water is 2264.705 joules per gram.
A joule is 1 watt for 1 second. (wow, that's easy) 
That means that the Instant Pot makes 1500* joules per second


* Your value actually goes here. 

Now you can compute: in my example it takes 2264 / 1500* = 1.51 seconds for the Instant Pot to boil 1 gram of water. 


1000 grams makes 1 kilogram. 
1 litre of water weighs 1 kilogram (not a coincidence)
A litre is basically a quart (good enough for this). 


So 1 litre = 1 quart = 1kg = 1000 grams = 1000 times as long as a gram, so that makes (in my example) 1510 sec. = 25 minutes.  Per litre.  

So if you have a 10 litre pot, that's 250 minutes or 4:10. 

Now you know how to set the timer on the Instant Pot. 

This will finish with some water left over, because we're ignoring inefficiency and the specific heat required to raise the water from tap temperature (15C?) to boiling temperature. That is small potatoes compared to latent heat of vaporization, and you need a little safety margin anyway.  

Also, you should be dumping out the last 20% of the water, because it has 5 times the mineral concentration of normal water, and if you boil it dry, it will leave mineral deposits on your Instant Pot.  That mineral action can actually etch stainless.  That's one problem that misting dehumidifiers solve (they throw the minerals into the air). 

Honestly, since the Instant Pot is so computerized, I'm amazed they allow any mode where enough joules could be added to evaporate all the water. It makes me wonder if you're tricking the computer to do this. 

What a bad idea. 

Metering water like you want is a rather hard problem

The crux of the problem being to stop overflows/runaways with a high level of reliability.  That is why you're not finding easy, off-the-shelf solutions to it.  Finding one that works at boiling-water temperatures is even harder.