I made this for my seed starting setup.  I have a heat pad under a capillary mat which worked well to keep things warm until I removed the humidity dome. 

The primary purpose is to provide a heat bubble/wind shield to help retain some heat and minimize excessive evaporation by encapsulating the sides of the plugs.  It also adds stability to a grid of peat pots in case they need to be moved around.  

Note: this is printed upside down and is intended to be used with the grid on top suspending the peat plug tray.  It is also made slightly short to ensure the plugs make solid contact with the capillary mat. 

Example peat trays: https://www.amazon.com/QDBNW-Biodegradable-Seedlings-Germination-Transplant/dp/B0DQP85ZPK/