This part has 3/16" crown on top left to right, which should be easy to form with a disk sander. There is no crown front to back, same thickness at front, middle and back. If you are making one by hand, form the top contour so the wood with the felt top pad will fit fairly snug in the space allowed.

Note: This part as drawn may or may not be original factory issue (maybe depending on issue date), but it does not exactly match the known original one in my MGA from August 1957 production. My original part is is curved (concave) on the bottom side from front to back, appearing to be steamed and mechanically formed to match the curvature of the bonnet. Or it may have warped over time from engine heat (speculation). It does have the felt pad on top as noted. It also has a horse shoe shape notch at front center to clear the bonnet latch stud. I may some day measure it for a drawing, but for now it cannot be removed from the car with the thick felt pad for radiator top seal glued in place.