The washers are larger than the ones that the OEM bolts come with.
I'd sooner go up in size for function rather than down in size for looks.
According to Rob Bell (Technical Guru over on the F-Reg) the ones I have replaced are not structural, and as our very own DD says - the sheet is quite thin around that area so larger washers are better suited.
I did have concerns over using Zinc plated as I had been told that they can corrode, but having read up on engine builds and being reassured that the originals were very probably Zinc plated as well i decided to give them a go. If they do start to corrode, I can always change back to OEM.
The spare wheel cover came as part of a set of 4 I got for a song on eBay - you can see it's part of a set as per the wheel marking graphic.
I simply split the set between other T-Bar friends.
Functional? - Possibly, it gives a nice handle to lift the wheel out with, but it's more to tidy the area up really.
I did take one out one of the Posi-Drive screws that hold the bumper on, but not sure on the size, so will be on with them once I find out what size they are - anyone???
Edit: I also have my eye on some nice black fasteners instead if chrome isn't your thing