I raided my stock of cardboard to make a template.
I was going to just duct tape it on since it only has to last the winter, but then decided to make ghetto brackets instead. They're plywood and they pinch the 90 degree lip in the wheel arch to give attachment points for screws.
They're actually very secure. I can move the metal lip by wiggling them. Wood sucks as a long-term approach of course; hunks of plastic or 3D-printed pieces would be ideal.
And the 2 brackets on the top of the wheel arch have to be much shorter for tire clearance.
This one doesn't come out from the tire quite as much as I'd like. There's probably only about 1 cm / 3/8" clearance.
Now to make one for the other side...