The clickfinity baseplates by James Boone are amazing https://www.printables.com/model/719455-gridfinity-clickfinity-baseplate-w-connectors, but they are not designed to be flushed with a vertical surface.
(You might also want to consider clickbase plates https://www.printables.com/model/982173-clickbase-a-no-magnet-latching-gridfinity-baseplat/files since they are a good alternative to clickfinity).
These section baseplates allow you to create a perimeter of width 2 for a total of size of at least 6x6. you need an original clickfinity baseplate (see https://www.printables.com/model/719455-gridfinity-clickfinity-baseplate-w-connectors) to fill the center (all edges should have female connectors).
For sizes smaller than 6x6, simply use a baseplate of the desired size.
How to use:
At this point you should have the perimeter. Now print the necessary clickfinity baseplates with connectors to fill the center.
A big thank you to James Boone for making available the Fusion360 file.
The author remixed this model.
Based on James Boone design and fusion360 files.