The stuff is easy to cut with scissors by the way.
Anyway, to obtain the curve, the panel needs to be slightly longer (by about 5 – 10 mm) than the length of the Unicorn HAT. Best to adjust this to your own personal curve tastes. You’ll need around 3 – 5 mm of material to form the arc* of the folded ends, and then about 8 – 10 mm for the retaining flap which holds it in place. (Mine is more like 11 – 12 mm).
(Hey if you do, please let me know how you achieved it!!)
*To form the arc (folding the flaps) just fold the material. I used a pair of pliers, but a bench vice would be perfect. Make several gentle creases so you don’t split the plastic, and you’ll need a slightly sweeping arc anyway, to go around the head of the screw.
I haven’t even tested it yet as it’s disappeared into my son’s bedroom…