There are a few ways to do it. One if vacuum forming, but it takes quite a bit of setup. And there is molding from resins, but also labor intensive. So the easiest way these days for a couple of one off covers is to have them 3D printed. At the most basic you will need a good drawing and someone can build a 3D file for you. If you know someone who can do the model for you then you only need to cover the cost of the printing which is very reasonable or it can be.
I think if you go looking down the 3D print road you will find what you need for a reasonable price. I could create a 2D drawing in just a few minutes and export to .dxf or .dwg if you need one. Those are interesting since they have the slight arch across the top. I don't know what that radius is but I think I could get is extremely close.
