There always seem to be unique methods used by beekeepers, to cut hand holds in bee boxes. This is what I use.
Clamp a block onto the fence. Note the four black lines on to of the fence. Line on left in C/L of arbor. The next three art block positions when cutting hand holds. Second from left is frons and back of nuc boxes. Next is front and back of standard hive bodies and supers. Third is sides of bodies and supers. If I want to cut hand holds in the front and back of a bee box, I clamp the block at appropriate line...where it is in the photo.
Place the bee box corner against stop and on table. Push box firmly against fence, and lower slowly onto dado blade.
Push box forward a couple inches to make the hand hold a bit longer. Tip box up and off of blade. Makes a nice clean cut. Not too fancy but does the job. I like 1/2" depth...a bit easier on the fingers.