First, before opening SketchUp and start designing, let's see if I don't find something allready made that could do the job.
I'm a real trashman so I keep every broken thing I have or at least keep good parts of it before throwing it away. I even collect what I find in other people's garbage.
It's a real bad habit because it turns my wife crazy !
But on the other hand I can allways catch something that I can reuse...

So here's what I found in my gar(b)age : 

The only missing part was designed in SketchUp.
It only consist of a small rectangle with round apertures on top and bottom.
Bottom aperture is same diameter as the garbage feet screw and the top aperture is same diameter as a Rapman foot.
Here it is viewed in SU with transparency :

And here are the final feet mounted on the Rapman :

Now my machine shakes much less when filling little gaps !
It's been a good improvement.