There are some very clever dongle enclosure modifications around, like these
I have never tried this, I suppose another way is simply wrapping thick tin foil around the plastic casing of the dongle and putting a few holes for cooling. It got me thinking though when I first came across some of the designs.
The main reason why i have not tried this is that I assume that using a power usb feeder cable for the dongle is prime suspect for picking up stray rf through the ground connection as normally these have a screen braid. The braid or the ground of the power cable feeding the dongle itself makes a perfect unbalanced antenna aka receiving antenna for broardcast FM stations and cell phone systems.
I can only guess that a metal enclosure for the rtl alone would add to the surface area of the “phantom antenna system” making it more efficient and adding to the problems of receiving unwanted transmissions.
Perhaps this may be improved by putting the usb extension cable and the dongle in a shielded box or use a metal enclosed dongle direct in the pi or somehow cutting or disconnecting the braid from making contact with the usb shell of the feed cable or ferrite beads may help a lot.
There are commercial dongle enclosures but I recall Some users of the ham it up hf upconverter were plagued by poor grounding contact due to the enclosure having a coating applied to it.
As said, I have not tried any of the above, just a few ideas to share.
Searching the net ,I have just found this interesting write up