Designing a custom part is easy: Just draw your component as a vector file. I recommend using Inkscape which can be downloaded for free from this link.

To make the component work with the grid in Circuit Canvas, set your document grid to 5px for schematic components, or 2.5px for layout components. And set the scaling to 1. All units must be in pixels.

I've written up documentation with all the details on how to do it here.