There are two versions of the top of the case, the v1 is designed for any heat sink that will fit on the second chip. The v2 is designed for the heat sinks in this kit: https://www.pishop.us/product/aluminum-heatsink-for-raspberry-pi-b23-2-pack/
Specifically, a 14mmx14mm and a 9mmx9mm
To install you'll need:
The back has a guide to prevent the case from rotating, as it's attached with only one top screw. It also helps to set it up at the lowest position on the frame.
The author remixed this model.