The Gridfinity system by https://thangs.com/designer/ZackFreedman is pretty cool and all, but one thing I haven't found (on like the first page of stuff because I have the attention span of a goldfish) is a base system for it that is modular. I've seen parametric bases, but not fully modular ones.

So, well, here it is. The connectors for the base modules might be a touch finicky to print because of the very small detail so be sure to keep that in mind. I put the clearance at 0.2mm because that's what I've found generally makes for an ok interference fit on my printers.

Each base piece is one unit in every dimension, including z (42x42x7) to make life simple for everyone. The test pieces I printed snapped together very nicely after I cleaned up some imperfections from the prints. I printed them in Inland PETG+ at 0.2mm layer height.

Update:

First version had incorrect corner radius on the base plate. This has been updated so that it now has the correct corner radius. There was no change to the connectors so those should still work, but if you printed V1 base you'll need to reprint with this update to be able to use with other Gridfinity modules.