It could be that some models are routed for a thicker plate than others. I only have owned neck-thru Alembics and they seem to have had pretty similar plate thicknesses. I know some basses had half-moon shaped covers and that not all covers are the same dimensions, but that seems easy enough to make sure you get one the right size. Any plating would certainly drive the price up if it were needed.
It's not a critical piece of the bass, though. A handy person might be able to get a piece of metal from a hardware supply store and make their own or even do something very attractive with some thin wood veneers or a small piece of hardwood.
I wouldn't suggest self-fabrication for a bridge saddle or replacement nut, but a truss rod cover seems safe enough if you just want something to cover the hole and look like it belongs. I probably also wouldn't think of doing this for my '04, but it might have been the right answer for the '82 I once owned.