A Gridfinity adapter is nothing more than the bottom mating portion with a flat top surface. This enables you to fasten whatever you want to a Gridfinity system without needing to 3d model anything specific.

For example, if you already have a great case for your tool, just glue the tool case to an appropriately sized adapter and bam, now it's a Gridfinity case.

Also included in the download is the Fusion360 .f3d file.