UPDATE: I now recommend to use Neels remix instead of this version. Printing separate connectors on their side and using them to connect baseplates works much better and has more flexibility then this design. There will be a stackable version too soon. 

You can extend these baseplates by snaping them together (it's easier holding them at an angle). I added little notches so you can stack them on top of each other on your print bed without them touching to print twice as many in one print.

For stacking you always have to pair an UP and a DOWN version and offset one of them by 21 mm in x and y in your slicer. If you don't want to stack its probably best to only use the UP versions.

I tested it with a 0.6 nozzle and super chunky printer settings in Prusa Slicer. So basically anybody should be able to print them if your printer prints well. 

This is the first prototype, I'm not sure if they clip together as sattisfying for everyone, as they do for me. I found it is easier to snap them together by holding them at an angle. If you press the male connector straight down it is more difficult and I'm afraid it might break for some people when printed in PLA. Feedback is welcome. I will update more sizes over the next days please be patient :)

Please hack the hell out of my idea and feel free to use my fusion 360 files (f3D). Im not super experienced in CAD this could probably have been made 1000x times more easy and elegant. Feel free to re-upload or distribute the files to other places✌