What you're really looking for is a 2-gang wall plate that has coax on one side and power on the other. Finding that as a single piece might be too much to hope for. The closest things a Google search finds for me look like this:

That is a 2-gang wall plate with "Decora" (the rectangular hole) on one side and regular (non-"Decora") power on the other, plus a Decora coax + Ethernet insert.
You simply need the same thing, except with a coax-only insert:

You should be able to find both pieces at Home Depot or Lowes, I think.
(If you can't find a wall plate like that, you could just get a 2-gang Decora wall plate and replace the outlet with a Decora outlet, but that's a dollar or so more expensive and will no longer match the rest of your outlets.)