If you're really set on using keystone panels, you could use something like this panel frame from Amazon

When I search for "inset keystone panel" or "inset low voltage panel" there seems to be some stuff out there.

Depending on the tools you have available, it might be easier to build your own inset panel. Below is what I did with the binding posts on a custom fiberglass sub-woofer enclosure I built for my car. It wouldn't be very hard to inset your posts like that using MDF or something.