You have a few suggestions already but my approach due to what I had on hand was aquarium tubing. Fit perfectly in one of the vent holes and its long enough to push down through the metal tray which holds it in place. If possible leave it a bit long and that way you can add or take away slack as necessary for opening and closing the bator.
I do the same thing. Aquarium tubing, with a large medicine syringe at one end. We suck the water up, stick in the tube, find a place to squirt the water, and voila. We also have aquarium tubing, a fish tank pump, and a heater set up to pump in warm moist air through that same vent hole. We have to move that out of the way to add water.
I also found it so much easier to control humidity when I added puppy pee pads under the vent hole. Underneath everything, directly on top of the water chanels. That way, if I squirt water in, it helps disperse the water and really has stabilized my humidity.
Good luck!!!
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