Compact Note Blocks simply allow players to use noteblocks when there is a top half slab or an open trapdoor above it, thus you are able to make more compact/creative ideas with noteblocks, in normal minecraft you can't which doesn't make sense which I didn't like so I made this.
Note this does not work with redstone, only player interactions.

Here is a comparison of the space you will save using this plugin.
On the left compact note block which is 2 blocks and on the right normal minecraft which is 3 blocks.