Slip around on top of the Greta you mean? Are all of the feet making contact with the top of the Greta? If they are, but it is a slippery surface, make it less slippery. Maybe cut some sandpaper squares or strips, and using a little hot glue (which is very easy to remove) stick the sandpaper to the Greta. This would give the feet much more friction to hold the MV50 in place.