I have a few terra cotta (whole) pots in several of my tanks. I siliconed (using aquarium sealant) a piece of plastic canvas cut into a small circle into the inside of the pot. This way, it's not visible from the outside (so doesn't look as tacky). (Especially if you can't find any clear stuff. All I had was a piece of blue lol.)
End up looking something like this. Sorry for the reflection and glare.....was a bad angle to take with my phone.
I highly recommend doing this...I was one of those people that had to deal with a fish injury from a smaller pot (one of the 1 1/2 " ones a few months ago. He decided to try to swim through it. He's double the size of the opening....he got stuck. The betta is fine, but I learned my lesson.