I believe the 3DR V2 shield is to be the best shielding solution available when using the mRo. It is cut to an exact fit and is only $15. There may be shielding solutions that are cheaper and work as well...but none that work better.
After typing all the stuff below, I just realized you asked only about shielding options, not GPS options. But since I typed it all...I'll just leave it...sorry.
The two most popular options seem to be the HERE GPS system and the mRo Neo-M8N system.
HERE seems to be more accurate, is more expensive, and is an external "tower" device that mounts to the outside of Solo. People seem to be divided on the look external mounted tower, and opposed to a 'non-factory' looking solution...but it's performance is excellent.
The mRo is not as accurate, is cheaper, and is a direct replacement for the factory GPS board. While the mRo is not as accurate as the HERE, it is definitely more than accurate enough, and many like it's clean, out of sight installation.