It was requested of me to divulge the secrets of my sloped handle cleats. I think the complaints about boxes being to heavy comes from the fact that people are trying to lift them from hand holds only. Those are not hand holds, those are finger tip holds. Finger tips are not made for heavy lifting. I add sloped cleats to all boxes.
I cut them at a 20 degree angle. They are about 1 1/2" tall. I brand before adding cleats. I screw and glue them on, I place their top outside low enough to allow lid over hang with their bottom above part of the hand hold. This allows me to stick a lot of my finger into the hand hold and get a real squeeze grip on the cleat. I can also lift empty boxes one handed, fingers under the cleat and thumb on the frame rest back edge. I usually use 3/4" thick stock, but made 1" thick this year cut down from 2X6 redwood. The problem with the one inch thick stock is that it put the medium supers just at their tipping weight when stood on their end. This would not happen with deeps. The ones with 3/4" stock do not tip. The slope also makes rain run off rather than behind the cleat to cause rot. End view, bottom view, whole finger grip view.