Epic Gardening 6-Cell Seedling Pusher Tool

This was created in Fusion and can be tweaked to fit your seed tray of a different brand as long as it has holes in the bottom to push the seedlings out. You can make the base thicker if you want but I found that 1/16" was sufficient. 

I have also uploaded a version for the  4-cell tray. 

https://www.printables.com/model/1136509-epic-gardening-4-cell-seedling-pusher-tool