I don’t use frames now, but I’d consider them if I could get a minimalist design that’s color-matched to my factory paint. (I’d rather not draw extra attention to that area with chrome.)
Make sure to use good stainless steel screws. It’s a pet peeve of mine when I see an otherwise-nice car driving around with rusty streaks down the plate because the owner can’t be bothered to spend a dollar or so on new screws.
You should consider having not only a frame around the edge but also a solid metal support in the back so that the plate lies perfectly flat and so that it’s less likely to be dented if some idiot backs into you while parallel parking. I guess that would call for a two-piece design with longer screws than the originals.
As already mentioned, you need to make sure you don’t obscure any lettering on the plate, including the state name, so you probably need to obtain and examine the layout of plates in every state.