How thick is the tab ? I am guessing between a 1/8 and 3/16 inch. And the main part of the cover ?
I would like to offer a suggestion, make the prongs for the back , tapped, and use a small contersunk Allen head on the outside to hold the prong on, then it will be flush. The prongs are fairly easy to make using a lathe and maybe mill. . If you can't make them, I bet someone here would do some trading..