That look good, just make sure the cover makes good contact with the main case. Also make sure the main case is making good contact with the copper surfaces.
Actually you don't need to go to this extreme, just making sure everything has a good electrical ground connection is sufficient, this effectively creates a somewhat like Faraday Shield around everything inside the case. You want all the parasitic capacitances to remain fixed, and a good mechanical structure keeps things from moving around.
You folks with these SMD fixtures should consider the DC Bias Adapter, kinda interesting to "see" how the DC Bias capacitance changes with ceramic caps, diodes and such!!
Good work and keep us up to date on your progress
Best,