What blocks are iron golem spawn proof?

Glass has worked for my farm so far I've changed it about 10 minutes ago after my golems were spawning on my half slabs changed it to glass and everything has spawned in the place I need it to including cats

STRING! They're cheap and effectively work on any block you'd like and do obstruct the 3 blocks of vertical space above any block that iron golems require to spawn. They can be stacked on top of each other as well, which allows them to be used anywhere, and don't clutter your view too much as they're virtually invisible.

Other blocks that could work, if you prefer them, are redstone dust, signs, repeaters, buttons, pressure plates, torches and comparators, for example. Essentially most non-solid blocks (I'm not sure about ALL) can be used to obstruct the spawning of the iron golem.

Using glass or chests as floor blocks also disallows iron golems to spawn, at least in 1.15.2 (according to my lazy testing that is, tell me if I'm wrong).

Edit: Golems cannot spawn on bottom half-slabs either.

Placing carpet worked for me! Glass block works as well.