Based of the work of Zack Freedman and nolanleot here on Thangs.
I include the Fusion 360 Files that also include the "blank" from nolanleot so you can more easily modify to your needs.
quote from nolanleot's:
These blank Gridfinity blocks are to help you create your own #Gridfinity pieces. They use the original geometry - The curves are all accurate and should match the rest of your Gridfinity set!
Currently in the mix: 1x1 Block 1x2 Block 1x3 Block 1x4 Block 2x2 Block 2x3 Block 2x4 Block 3x3 Block
All blocks are 1.25" from base to max height. All M3 screw holes are 1cm deep from the base.
Gridfinity Tips from Zack Freedman! No brims or support required! Use any filament with an 0.6mm or smaller nozzle. Wider bins can curl, so maximize your bed adhesion if you're going big! All storage blocks are compatible with all baseplates! Most storage blocks are stackable! You can add 6x2mm magnets to the corners! They won't magnetize the contents. You can also use M3 screws in the corners.
https://thangs.com/designer/nolanleot/3d-model/Gridfinity%20Blank%20Blocks-211251