I have two different methods:
I got a black laundry basket at the Dollar Store, and put a potted lily in there, along with hornwort and anachras. Small fish can get to it, water filters through so the plants can do their job cleaning and oxygenating it, but the koi can't get to the plants.
And I also have an open top rectanglular basket made of a PVC pipe frame (drill holes in the PVC, so it doesn't float), with black plastic mesh (like for a screened porch or patio sun shade) sewn around it. In one of my rare successful attempts at posting photos, I did get one of this basket posted, somewhere, about a year or two ago. I have lilies, floating plants, and delicate fantails, orandas, and goggle eyes safely sheltered in here. Fresh water flows through easily.