In some corridors the spawners can actually spawn mobs outside the walls if there is a ledge just beyond the walls.
I would suggest to put slabs on any surface that is not normally accessible to the player.
That way you can keep the 'empty' feel outside the walls while making it spawn proof.

nvm, CurseForge messed up the version.
It had downloaded the latest version but was still using the previous one. :/