So I tried putting the gutter at a 90 degree angle to the box, creating two separate paths, one for his approach to the box and one for his departure. Didn't work: he used the departure path for his approach as well as his departure.
Ok, back to the idea of something more knee-ish. Having no spare knees around, I grabbed the next best thing: a music stand. I put it so that one of its feet slid under the box to stabilize it so that it wouldn't get knocked over if Walter should brush against it. The gutter went in its usual place.
Well, holy moly! Instant boxturn transformation! I was amazed, thrilled, and perhaps even stunned as I watched his first try at it and saw the old, flowing turn he did once upon a time many, many moons ago. Unfortunately, with the box sprung and his new style, his mouth isn't in the right place yet so the ball was shooting out way in front of him. So, I unsprung the box. Success!! His next try, same thing! Woo hoo! Grabbed the camera to get some video of this momentous occasion. :-D