The TravelCNC already has an inner lining of Basotect acoustic foam on the inside.
(Not all of it mounted yet because I'll improve some of the cabling hidden below the foam.)
This helps a great deal with high frequencies.
For the low frequencies I have a heavy, 10mm thick rubber sheet below the actual machine.
It turns out that this is not enough. I get a great deal of vibration that can actually be felt on the wooden stage box.
Next idea: there are 4 bolts going through the rubber and mounting the machine to the case.
I'mm try to widen the holes in the wood and install sound insulating bolt anchor fittings to insulate the bolt from the hole in the wood.
...let's see of that helps.
I finally finishd the sound proofing