Throw a Cushion on a Sulfur Cube to make it mountable!

Works with Small Cubes too

To remove the cushion either put the cube into a bucket or shoot a projectile (arrow,egg,snowball) to destroy the cushion.

Known issues/limitations: Its not possible to make cushions not pop off when not on a block so they pop off every few seconds for a tick. When multiple cubes hang out close to each other the cushions could move to a diffrent cube because of this.

If you really want to its possible to place Cushions not aligned to X and Z grid - just right click with a cushion on a Sulfur Cube that already has a Cushion on top of it.