I think it would look best to go with a custom cut adapter plate that fits in the depth between the mud ring and the drywall...so that you could use a 2 gang plate like Daniel posted above. I think this will look the cleanest. You could measure the opening and have a local metal shop cut you 1/16" plates that you could pilot (for the 4 screws to attach it) and tap/thread 2 center screws for your decora screws.

I did find two different adapter plates that require no Kustom work. These would get mounted then your decora device monted over them...then your decora plate over that. So it would be stacked. This wont look perfect but it should be fine. Maybe go with a screwless single gang decora plate to clean it up.

[Link: alphacommunications.com]

[Link: clipsal.com]

I would probably order one of each to see which A.Works B.Works/looks best.