The Creeper Fountain is a proof of concept of funneling slender mobs (zombies, skeletons, creepers, etc.) from four 2-wide water streams into a central 2x2 bubble column. The design is efficient at actively pulling the mobs into the column and does not congest even under heavy load. It has been stress tested with 100 creepers in this world upload.

How it works:
The ground floor consists of only two, diagonally placed soul sand blocks, with air blocks next to them above the amethyst blocks (those are only placed as markers). Because of these air pockets the water sources placed above the soul sand can not form a full 2x2 layer, but flow into these holes. This creates a flow that pulls the mobs into the column. The other two soul sand blocks are placed on the other diagonal, two block above the others. They guide the mobs and complete the 2x2 bubble column.

Each water stream is funneled into a 1-wide entrance. Be aware that the water stream leading to the entrance needs to be flowing diagonally, i.e. there is only 1 water source for the 2-wide stream.

This demonstration build is based on my older "4-Way Water Column Entrance for Mobs" design for 1-wide water streams. The Creeper Fountain makes heavy use of barrier blocks to improve visibility.