Use these posts to stack SSendam's paint holders (link below). They fit into the four corner holes. You should be able to stack more than two racks as there is enough clearance within the holes for two posts. These are just tall enough to allow for the tallest Citadel pots to fit without touching the roof.

Paint holder: https://cults3d.com/en/3d-model/various/gw-paint-pot-holder-for-really-useful-box-3l