I have just a card underneath, say 2mm. On top of the lid I have a 5AH lead acid battery, 1.6Kg. That is so there is no movement when keying, just like when screwed to the bench as was always the case. Flexible feet would ruin it. It just floats along at 23 wpm, but I need more practice to get back to 28's no errors. My favourite key.
By the way, I replaced the sleeve bearing with new roller bearings to take out a slight wear. What does yours have Gavin? I also used paint stripper to reveal the brass.
73
Andrew