This things fit in my small green house. The Base grid is 190 x 134mm. The bottom is removable, so you should be able to push out soil and seedling. The pot has an little handle to simplify removing of a single pot. On the bottom of all parts are holes for drainage.

Key feature was for me that i can handle the pots seperatly, as I tend to mix everything in the green houses.

I printed it with petg. The pots with 3 side layers, as I want them to last for a few years.

Tell me if you need other dimensions.
Would be happy to see your makes.

Edit 10/18/2018: Added a little tamper to easily compress the fresh soil.
Edit 22/11/2018: Seedlings are growing pretty well. Moving small pots around works great.

Any suggestions how to contine pot design, as the plants are hopefully growing?

The greehouse on the picture should be this http://stewart-garden.co.uk/products/black/22cm/unheated-propagator/