I purchased a Mission Engineering SP-1-ND expression pedal. When I went to go mount it on my pedalboard, I discovered that an additional accessory was needed to do this. Not wanting to wait another week for this to arrive, I designed my own. 

I made mine with 6mm acrylic that I cut a laser cutter. I chamfered the holes on a drill press so the screws will sit flush. However, I think 3D printing would work as well or better, and as a bonus it requires no post-processing on the holes.

To install this, you will need to remove the feet from the pedalboard. Do not re-use the screws from the feet; they are too short. You will need some #6-32 x ½ inch screws. You can find these at the big box stores, McMaster-Carr, Bolt Depot, etc.

Note that the design is deliberately inset a little from the edge of the pedal.