Custom Block Outlines (CBO)

This project is deprecated and will not be maintained or updated. Unless I feel like it. But I probably won't.

This is a Fabric mod that allows you to customize blocks' outline shapes/collision boxes. You can either specify another block to copy or create the shape yourself with cuboids. 

How it works:

When you first start the game after the mod is installed, it will create a 'outlinepacks' folder in your run directory (the .minecraft folder usually). This is where you put outline packs.

An outline pack is just a folder containing a bunch of JSON files, one for each block that you want to change the outline for. The name of the outline pack doesn't matter; the name of each JSON file is the id of the block (e.g. brewing_stand.json or redstone_wire.json). Below are examples of how to fill out the JSON file (sorry about how the JSON looks, apparently CurseForge doesn't handle indentation well).
 

Copying another block:

In poppy.json:
{
  "override": {
    "id": "minecraft:spruce_stairs",
    "blockstate": "facing=north,half=bottom,shape=straight"
  }
}

{
  "override": {
    "id": "minecraft:dandelion",
    "pos": [24, 65, -121]
  }
}
(the blockstate tag is optional)

Creating your own shape:

{
  "cubes": [
    [0, 0, 0, 8, 8, 8],
    [8, 8, 8, 16, 16, 16]
  ]
}
Result:

btw, the same packs need to be on the server and client if playing on a server. It won't crash or anything, it'll just be really glitchy and stuttery.

FABRIC API REQUIRED