The MDF waste board of my Shapeoko 2 was starting to sag due to humidity so i made these little "feet" to keep them flat. They're exactly 20mm tall and can be attached to the bottom of the waste board with countersunk wood screws.
The STL is solid. I find that with PLA, even slicing it without top or infill (as pictured) works just fine.
I'm using M4 x 16mm screws as part of the Ikea wood screw set.