Ok, I edited in the rough position of where in the system they would be positioned. Essentially putting a half a piece of pipe inside your nft tube between the elbows, this would allow the water to clog back to it's nearest elbow for a flow channel of nutrient solution, but would provide an overflow that to the next elbow in succession thus preventing overflow at the plant sites. The key is to make sure the intake of each section is before the first plant site of that section and terminates after the last plant site of that section to not be obstructed by the main root mass of those respective plant sites. If you use the thin walled white pvc for sprinklers you will have flexibility and can cut with shears or tin snips. I would probably use 1 inch, then use silicone for adhesive and just clamp them in place while they dry. Shouldn't have any problem sliding in the half pipe into an end plant site of each section and clamp using the plant site opening.
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