There are some EMI shielding cages, tape, fabric etc etc.
But you need to DIY a bit, and they are a bit expensive
Keep in mind, that deending on what frequencies the EMI shields protect against, it could and could not work for you.
I'm a bit surprised, but most don't actually think of making an EMI shield for their amps that have this problem, but rather buy new ones with good EMI shielding built in.
But EMI shielding materials MAY work.