I was able to do some research and unfortunately I couldn't find a suitable softbox. Furthermore, compatibility with a softbox is not mentioned in any of the manufacturer's documentation, although I assume that the "diffuser disc" included in the scope of delivery is intended for this purpose.
This is an area light, which emits soft, diffuse light over a relatively large area. To be honest, I don't think there is a softbox for this. That would be double. There are many other products from Walimex with an even larger surface and correspondingly softer light. Don't forget the distance! If you move the small flat very close to the subject / model, it is a large surface. Relatively / in proportion. Distance is just as important as size