Gen. 1 and 2 are about in the middle. Gen 3 doesn't look practical but I didn't peel off the stickers to check for a second screw on the back of the chassis.
Also keep in mind that these are fanless and have vents in the top. Try to leave 1U of space above them for ventilation. I'll often mount them directly below a cable management panel or blank if the rack layout allows it.
