I needed a Gridfinity insert for the IKEA KUGGIS 18x26cm box. I was inspired by Justin Levine's insert for the larger KUGGIS (https://www.printables.com/model/357454-gridfinity-for-ikea-kuggis-box).

The small and large ones that I have, have tabs in the corners at the bottom, so this model has cutouts for them. They keep the base plate solidly in place so it doesn't need to go all the way to the edges.