I'm having an iron golem farm.

Some of the walls were made of double half cobblestone slabs (bottom and top one), to form a full block.

As a result, golems consider that block as not full block and allow spawning within it, when the bottom one is a normal, full (spawnable) block.

The new golem just continue wandering within the row of double half slabbed wall, like it's not a full block. (Breaking the farm, due reached golem limits)

I haven't checked if the golems would be able to breathe within them too, as my walls are only 2-high.