I set up a 'pretty easy' way to grow duckweed. I just used a 'kiddie pool'. I ran 3/4 inch line, coming from my swirl filter. On the end of the pipe, going in the pool. I added a slip cap with a couple small holes drilled in it. I just wanted a small flow of water.
The drain, turns down to the bottom of the pool, so I don't lose any duckweed.
My bulkhead for the drain, is a male and female adapter, gray pvc conduit( electric).The threads are straight, plumbing pvc fittings are tapered. I used a little silicone for a seal.
The overall cost is pretty low for this set up.
If you use gravel grow beds, you could run water to the duckweed... 'after' it goes through the beds. That way you don't have solids collecting in the pool.