This box has generic size, that could be modified in FreeCAD parameter table - see image. It is recommended to use FreeCAD and resize a box for your needs.

Gap 0.7mm seems to be too big on my printer. 0.5mm gap seems to be good.

It is recommended to use brim for printing. No supports needed. Because of model simplicity you can print it as draft it will save time and a model would not be affected loosing of details.