TV Sound Bars are designed to be used at further distances, maybe 10 ft. However, many would like a sound bar as a PC monitor speaker. This set of risers angles your sound bar, so the speakers are pointing towards your ears, not your chest or belly.
It is recommended to have ½ inch felt pads or silicone bumpers to be installed into the holes/pits. They will both serve anti-slip and provide a cushion between the riser and your sound bar, to remove undesired vibration or noise.
Here is one for reference. Not affiliated, it's just a common model.
Any foam tape or acrylic tape would work just fine.