The Lego Digital Designer is an awesome tool for designing a new creation on your computer and creating a build guide. You can then use the Lego Pick a Brick site to buy all the bricks you need. A Raspberry Pi case can be put together for ~$20.
There are some caveats however:
- You can’t automatically generate a shopping list from the LDD — you need pick the bricks manually, which can be a tedious exercise.
- To rub salt into the wound not all the bricks in LDD are available on the Pick a Brick site — so you need to switch back and forth between LDD and the Pick a Brick site to double check you are using bricks that you can actually purchase.