And I test the fit again. I can get the thing around the wheel without obstructions. Nothing is rubbing! But the tabs don't reach deep enough in the frame. I think it's a metric vs. inch thing. My frame is made of 3/4 inch boards. Who knows what Matthias used in Canada? I don't. But I need to get about 3/16th of an inch closer and I can't.
I need to cut these notches in the cover sides deeper. But then it won't fit over the wheel any more. The front of the cover will hit the front of the wheel. It won't be deep enough. So I think I'll need to cut some strips to glue to the edge of the cover to give me the depth I need, and then cut the notches deeper.
So that's where I am. I haven't touched this project in 8 days.
It's almost to the point that I'm considering just building the cover over from scratch again instead of trying to shim my way to a good fit. But I used my best scrap of plywood, and it looks so nice.