Step 1: Acquire a water holding device.
Step 2: Apply duckweed
Step 3: Wait a month
Step 4: Harvest duckweed
If you try really hard, it won't work. IMO just set up a 5g with some gravel, throw a few plants in (like anacharis or hardy things you have in other tanks) and add a few fish like guppies. Do whatever you want to it, or nothing at all. You can overfeed, not feed at all, either works. When my water got mucky the duckweed stopped growing, but when it was nice and clean it grew well. So keep the water clean (by that I mean clear, not green, no algae) and then add some duckweed and wait.
In California around this time, guppies and everything else you want should be fine outside in a 5g for a few months. SoCal is even better than NorCal, lucky for you. If you live nearest the ocean in Inland Empire, then you should be able to everything other there for the entire year as long as it doesn't get below 50F at night. My guppies survived 40F nights, 100F days at the most extreme. The water stayed about 55F to 85F at the extremes. No food, mucky water, still reproduced.
Like seriously, if your friend has a pond on their property that they will let you put some duckweed in, then throw some in and go back to collect it in like three months lol.
ALSO, it may be the source of where you're getting your duckweed, and there may be different kinds of duckweed. Get duckweed from a few different fish stores or hobbyists and try that out.
Not sure if it matters or if you've tried it, but maybe dechlorinate the water if you haven't already. If you're using tap water, it may be something in it. Try distilled or bottled water from the grocery store.