I just did this to my back row of pedals. My Rat was tall enough, but three of them needed 1/2" risers to help clear the knobs of the front row pedals.
Home Depot, 1/2" x 3" (actually 2.5") x 24" strips, chopped to pedal height. I think they were poplar or pine. Spray painted black, screwed down to the board and tops velcroed. Pretty easy.
The Peppermint, Elec Lady and RS Chorus were too low.