It is, I believe, slightly arched toward the outside edge. It will be folded over on the front and flanged on the other three edges, with the inside edge rounded over and the outside and rear flat at 90-ish degrees.
Here's what a real one looks like, flipped over (as stolen from type550.com):
Gonna re-check the front-rear profiles tonight and transfer them to the edges of the 2x lumber. If the shape is as I think, I'll dig out my dado blades and run that last board on my table saw six or eight times, adjusting the depth as I move across to get the curve right, then finish with the belt sander this weekend. Pretty sure I'll want a solid or near-solid buck for this.
If it's curved like I think, the rear flange would be the only tricky one—and should not be too hard to fold in since the area under the panel is completely open. Think of it as a pleat. You could hide a couple of them under the mounting plate for that far pin and they'd be basically invisible.
Or (and!) before I even set it down on the buck, I can roll the whole panel in my little bead machine, using two rollers with no bead, to stretch the top just a little and make it want to curve, as in an English wheel. I've also requested a set of spoiler dies for Christmas, which could impart a pretty major curve across the panel directly if needed (it won't be though).
Thanks, @Gordon Nichols, for reminding me of Schelin. If I still lived in Fairfield I'd probably indenture myself to him for a bit. I will reach out. There is certainly an order of operations to this that will make it either comparatively doable or damn near impossible. Maybe he'll tell me.