You have a couple of options. You can look under the DIY section on the TFK forum for a plant corral design (really cheap and easy) to keep the duckweed at one end of your tank or you can get a sponge filter (just the sponge) and cover your filter intake or wrap the intake with the blue/white "filter floss sheets" and hold that in place with rubber bands.
It all works, just a matter of what looks good to you.
Filter bags work, but seem to clog quickly for me.
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