Should a diffuser be open or closed at the end? It seems to me that if it is open then all the smoke would go through there and not the slits/holes because that way had a wider opening and thus is "easier" to go through.
Well not really ha it should be open, the smoke travels down the diffy, so it hits all the holes/slits first, and whatever is left goes out the back. It would work more like a closed perc if they didn't leave them open, meaning more drag.
If there are slits, there should be a hole in the end. If it is holes, then the end should be covered in small holes.