Place Anything On Half Blocks

Build beyond Minecraft’s usual block grid with Place Anything On Half Blocks. Place blocks directly on bottom slabs or beneath top slabs for a snug fit without the half-block gap.
Combine different slab materials, add decorations to slab surfaces, and create builds at half-block heights. Placed blocks keep their original identity and behavior, with collision and selection adjusted to match their position.

Required on both client and server (if you are running a server. If not and you are playing singleplayer, then just drag and drop it into your mods folder like all other mods)