Here is the render. The orange thing is a one-sided square with a 10% fillet.

Below that, on the left is a panelled box with the hard edges. On the right, the same thing, but I added the calculation I gave above for a 1% fillet. It's clearly better. I've added this to the shader and you can dial in any k you want.

I'll also add chamfer to it - that's trivial.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)