I would buy 2 more 380mm 2020 and build a cube
Bottom
Using 4040 would stand tall 2 X 420 front to back
Using 4040 Would Stand tall 2 X 400 Right to Left
top
Using 4040 would lay flat 2 X 420 front to back and have a 20mm overhang both sides
Using 2020 Would use 2 X 400 Right to Left
with the 4 X 380mm would attach top to bottom
inside dimensions 400 X 400 X 380