TinkerCAD is a free design tool that works via the web. Great for Chromebooks and beginners. This lesson helps you design the sides of a cube. The challenge is to master the skills, then come up with a solution for the bottom and top of the cube or box. My students are working through it now, so I will have more tips and videos on the way.
If you have never used TinkerCAD before, here is a link to it.