Cubic tanks

I've seen that tank in a video where a youtuber found a "tank" craft and tried to launch it to space and decided to put a tank right under the craft, but it became square! I quickly found that craft and saved the square tank for use in builds, but i still cant replicate another thing that he did: he had 3 completely different sides. If i try to change a side it goes back to normal and doesnt save the shape. Its not a bug report really, but i find that really strange and decided to ask if anyone knows how to make that tank myself and how to change its sides freely.