I don’t think you can shield an adapter. It’s basically a power conversion transformer. All you can do is change the shape of the magnetic field that surrounds it so it does not mess up the signal in any of the wires your signal goes trhough. And you have this power wire going into your effect where the signal is.
A filter circuit might be possible.
But the easyist solution would be to buy a couple of rechargable 9v bateries and always have a charged one handy becuase they don’t last a long as the regular batteries. Maybe in this age of powerful nicads there is an even better solution. Like you could power everything with a battery and avoid all the hum. but you may find that the hum adds to you sound.
I have been reading a book titled AMPS!. There are amp builders that seek out special transformers becuase they effect the sound of the amp. Aspen Pittmen of Groove Tubes made the Trio Preamp that J supposidly uses. It looks like it has two different powersupplys: one for a Fender sound and one for a Marshall sound.
It’s all a little too much for me. I just use a tape deck as a preamp. This hum is all part of the internal feedback that builds up. some where in there the hum turns into a scream!
If you want to learn something about electronics that is important to electric guitarists read about inductance. THAT’S YOUR HUM!