This is a small V-Block, which is designed to help index, and hold round materials. This block is a 3" Cube, but can be resized to fit your needs. There are holes on the center of every side for 1/4-20 bolts.
V-blocks are used for holding and indexing round parts for milling or drilling. While these blocks aren't as rigid or precise as a hardened and ground steel version, they are fine for low precision cross drilling, and filing, and finishing.
This is the metal version of this thing: https://amzn.to/2XgKxBr
This is the filament I use: https://amzn.to/2MSLBYk
This is my printer: https://amzn.to/2MSLBYk
I've included the original CAD files, feel free to use or modify them however you want.