There are various tools that you can use:
- Shearing is quick and easy, but significant shearing results in 'thin' brushes.
- With the clipping tool (shift+X) you can start with a larger brush and cut away parts until you get the desired shape. Press shift+X again to cycle between modes (remove right side, remove left side, keep both sides).
- With vertex manipulation (shift+V) you can move each vertex or edge individually, create new edges (by selecting two vertices or two edges on the same face and pressing ctrl+F) and remove vertices (by dragging them onto other vertices). This is a powerful tool, but keep in mind that each face must remain perfectly flat - bent or curved faces are invalid. Brushes must also remain convex (e.g. no U-shaped brushes). Press shift+V again to cycle between modes (vertices and edges, vertices only, edges only).
- When creating a brush, you can choose between a few different shapes. 'block' is the default choice, but 'arch' is probably more useful in this case. Keep in mind that it'll be facing towards you in the 2D view that you most recently clicked on.
If 'hollow circular object' means you want to create a curved pipe, those can be tricky and time-consuming to do by hand, depending on how many segments you want. I usually stick to 1 or 2 segments, with 1:1 and 1:2 shearing. I also keep vertices in the straight parts on a 2-unit grid, so 1:2 shearing and clipping won't produce invalid brushes. There are also a few tools out there to generate curved shapes, but I'm not sure if they can do hollow pipes.