Difference between 洞 and 口 for holes in things
In my opinion, this is a &quot;口&quot;:

This is a &quot;洞&quot;:

口/口子 means &quot;(人体或物体的表层)破损的地方[the place where it is damaged(on the surface of a human body or an object)]&quot;, 洞 means &quot;物体中间的穿通的或凹入较深的部分(The penetrating or deep recessed part of the object)&quot;. You can say &quot;我的手破了一道口子&quot;, but absolutely cannot say &quot;我的手破了一个洞&quot;! Or I can't imagine how panic the person who hears it will be.
Some other examples:

日子久了，水滴竟在石头上滴出一个洞来。(水滴石穿)

他的手被小刀划了一道口子。