On my own board, I obtained the better result glueing the brass plate directly on the fiberboard. Since I use high speed motors and light weight counterweights, I obtain a game really smooth and not "bumpy" at all on 5 to 9/10 power level adjustement.
When I experimented double sided foam tape, I had to make bigger counters weight, increasing the "bumpy" effect also.
Removable fields is really interesting, but I don't see what I should do to make it eficient too...
