Simple triangular masu box lid

Long title but simple model, it's my first model and diagramming so don't be too cruel :P.
Note that this starts wit a square (4 sides) and ends with a triangle (3 sides). You can also start with any n-sided polygon and you will end with a (n-1)-sided lid! But not very useful cause I didn't know any way (but with modules) to make a pentagonal box.