I think I understand.
First let me say that you should *always* avoid sharp corners. A true rectangle will be problematic in any geometry. At least use the biggest fillets you can at the corners; best would be to use a slot with semicircular ends. (The best slot of all would be a cylindrical hole).
Second the bottom line of what follows is that the slot would best be in the x-y plane as you've defined the geometry; ie. the compressive load should be perpendicular to the slot.
Third, a confusing attempt to describe the geometry as I understand it follows:
There is a plate in the y-z plane of thickness x. There is a compressive load in the z direction.
You are going to stab through the plate with a knife.
Shear stresses leading to shear failure of the plate will be a minimum when the knife blade's two axes are perpendicular to the plate's z axis.
Other geometries are hard to describe in a crystal clear fashion as there are six axes involved (3 for the plate & 3 for the knife.)
Perhaps the simplest way to think about it is to imagine the knife blade (or stab wound) like a long envelope lying face up on the desk; the short direction is the width of the blade and the length is the depth of the stab wound (length varies depending on how the blade is tilted with respect to the plate when the stab is made.)
Now put a pencil point in the center of the envelope and hold it in the vertical position with your finger on the eraser; this represents the loading direction. You can tilt the loading direction along the envelope's long axis or along it's short axis.
Tilting the loading direction around the blade's long axis increases the shear stress component which reaches a maximum at 45 degrees. Such tilts don't affect the depth of the stab wound (the length of the envelope.) In a fracture mechanics sense such tilts result in Mode II loading.
Tilting around the blade's short axis also increases the effective shear stress component (but not as rapidly because the depth of the stab wound increases as the angle increases). In a fracture machanics sense such tilts result in Mode III loading.
I'm sorry for the complex description but the geometry is difficult to describe in words and precise mathematical descriptions aren't very useful in visualizing things for me.
I hope the above describes your situation in an understandable way. Are there other constraints on the allowed orientation of the stab?
The ratio of the plate thickness to knife width is a detail that might matter. I've assumed the knife width is big compared to the plate thickness.