My NSM 10's came with these rubber strips that I stuck on the sides ( bottom) It's not much but it results in the box having a 1/8" space and the rubber is the only thing sitting on the shelf which is soft Pine. The speakers front face is just beyond the edge of the shelf too.
There's another thread going here where the OP has trouble with reflections from his desk. I don't really have that issue for a few reasons.
There's a lot of gear on the desk. There is not much clear surface when I'm working. Controller, Keyboard, Mixer, audio interface with a Pre amp on top, note book, guitar picks, microphones, capo on and on. The perfect random defuser!
The desk is soft Pine not hard at all. I avoid polished hardwood or Formica.
The speakers are about 2.5 feet from my ears. At 75-85db they are going to dominate the room.