Mulberry (Mullbery) Metal products used to have a cast aluminum bubble cover that was inpressive looking, and pretty sturdy. They may still be available. Most of the supply houses carry Mulberry Products, they make all kinds of device plates, FS boxes, 4" sq blanks, etc. Check your supply house.
Mullbery is in Union, NJ, and I don't know if they have a web page, but a search engine will tell.