I am mostly certain I am not understanding the math here but in my mind a rounded_box should keep the same outer dimensions but have the corners and edges eased by a radius.

this is a contrived example I made to show the difference, I would not expect there to be a leftover band around the object.

from sdf import *
w = 50
l = 25
thick = 3
base_rad = 5
base = box((w,l,thick))
base -= rounded_box((w,l,thick),base_rad)
stl = base.generate()
write_binary_stl('test.stl', stl)