Pressed steel...
It's formed thin gauge steel, with some rubber flaps at the edges which I didn't include in my measurements. Slots where it fits past the handbrake cabling.
I think TMH sells one in "GRP" meaning glass-reinforced plastic, or what we call fiberglass here.
Seems it would be easy enough to fabricate with aluminum, especially if you know someone with a sheet metal brake or some other kind of bender.
The general curve is larger than a tire's curve... I think you could approximate it with some clever thinking and effort.