I always had a hard time finding a litter box that was deep enough for my cats. They kicked loads of litter out the side with traditional boxes. Then I tried a covered box and it just made them stinky when they came out. So I made my own litter box.
By cutting the entance so big and so low, you're still allowing litter to be kicked out of the box. I suggest that you don't cut a hole at all.
I use a large file box without the lid, and pour the litter into the box. I have a 23lb male cat and he is able to jump in and out, leaving the litter inside the box.
I always had a hard time finding a litter box that was deep enough for my cats, so I made my own litter box.