terra cotta plant pots come in many sizes.you can carefully crack them,file them at the edges,to get to size opening you want.also i have created many things using aquarium sealant,slate pieces formed into any size
sorry i don't have pics.that was from my african tank years ago.i bought slate tiles,you can find big 12 inch or the small 4-6 inch size.yes all aquarium sealant is safe,let it cure 24 hours or more.