I used a sheet of aluminium about 300mm by 100mm. The shape was determined by trial and error using several cardboard templates. The surfaces that bear against the binoculars are not in line with each other so that when the shade is bent to shape it slopes up towards your forehead.
The most essential feature for your comfort and safety is to have a secure rubber edging. I used PVC beading from Maplin, part XR78K; a standard length of one metre is adequate.
I attach the shade to the binoculars using rubber cord (from a yacht chandlers) looped and hooked through the holes. There are even eyelets in my holes!
To prevent reflections from the aluminium I used sticky-back plastic that I already had: matt black inside and wood-effect on the outside.
For the finished item, see the other page.