I have a basic plugin that sees if you have a slab in hand and if its named "Upside down half slab". It then flips the placed block from a normal slab to and upside down one.

The problem is when I move the item from the inventory slot it was added to the mechanic stops working. Which I am unsure of the full reason why. I will attach the plugin so you can test it your self. I will also point you to its source on github. https://github.com/chibill/Slabs/tree/1.2.0

I am using this version of my plugin code because purrur half slabs don't seem to behave like normal half slabs code wise. (My dynamic slab detection uses reflection to detect all slabs and it seems to have broke.)