This is a schematic for a full chunk of cactus. This build will not work in a vanilla Minecraft world. For this farm you need hoppers that suck items from the entire chunk. The server that I made this for has custom hoppers that grab the cactus and sell it automatically as soon as it pops off.

Here is a link to the video where I explain how to use it.
youtu.be/J4Ord8Vd62g