4" by 3" gutter downspouts from Lowes are great if you're looking for hydroponic nutrient film technique (NFT) growing channels. 

However, it comes with the challenge of closing off one of the ends. This is my solution. A very simple end cap. 

Before beginning, ensure that the downspout has been cut cleanly vertically, not at an angle or with imperfections in the edge which can improperly seal and lead to leaks. 

To use, fill the channel with food-safe silicone adhesive. Then press-fit the cap onto the end of the vinyl downspout. Allow to dry under compression according to the directions of your silicone.