I have a large zoozone and actually used the hole to fit a bottle! Ths one of the flat backed bottle/curved ath the front and is originally designed to sit on the outside of the cage. I put the white bottle holder through the hole and affixed from the outside (it has a pin and screw attachment). Because the bottle was designed for outside the spout was pointing in the wrong direction, so I got some hot water and dangled the plastic lid (with spout) into the water (just the plastic bit) and then grabbing the spout with a tea towel I turned the spout 180 until it was facing the curved side and not the flat side.
Now the bottle sits I the cage and the hole is filled. It was driving me mad too. I hope the pictures make a bit more sense then me trying to explain lol.
Photos below.
For the moment I have kept their old block built bottles until they are totally ok with the new bottle. I don't know about you but my cinnamon can find a hole in anything lol.
Love the cages by the way :0)
Arraminta