What could I call a hole that is shaped like an octodecagon (a figure that has 18 sides) in a less sophisticated way?
Can I say 18-facet hole/opening?
It's an 18-sided hole.
I don't think the word "octadecagon" is sophisticated but I think it would sound pompous if you used it in normal everyday speech. Most people wouldn't know how many sides it had so if you said "18-sided" no-one would be confused.