It’s a little bit tricky to get the lamps to shine only at an angle – the LEDs distribute light over a full 180 degrees. There are two lights in each “gap” between diagonals. The bottom LED needed to illuminate only the upper diagonal, whereas the top LED needed to illuminate only the bottom diagonal.
So I did quite a lot of experimentation with small pieces of black card, as you can see here, until I had shapes that blocked direct light to the nearby diagonal but lit the nearer half of the far diagonal. (There is an LED both sides, so each one need only light a half.)
Then I created a set of shades using fairly stiff plastic. This was so I could bend it and it would stay in the shape I had created. I spray-painted these black and added white paper where reflection was necessary. I glued the shades to the inside of the frame using Copydex.
Actually lining each shade up accurately was quite difficult. At close range to the lamp, a few millimetres can mean the difference between shadow and light.
Finally, I added white paper along the top and bottom to expand the effect from the one LED at each of top-left and bottom-right.
The light clearly comes from individual sources – more-so than expected, and reflection from the lit diagonal illuminated the unlit diagonal. But overall I’m quite pleased with the result.