I built my first lego case about four years ago roughly following the creation of Biz, a 12 year old girl at that time - https://www.raspberrypi.org/blog/bizs-lego-case-the-borreaton-scouts-hit-germany/. At that time it was hard to get a enclosures for Raspi at reasonable price, so users created their own.
Needless to say, there are also lego-compatible case on sale which can be integrated into lego applications, e.g., to replace the original mindstorms brick, e.g. http://knowhowtec.de/lego-kompatibles-gehause-fur-raspberry-pi-2-modell-b-b.html¤cy=EUR&language=de
"It always takes longer than you expect, even when you take into account Hofstadter's Law.", Hofstadter's Law