This is a howto for a simple bracket object made from a slab. This technique is sometimes called 'slabify'. Keep in mind that anything you can make in Archicad can be saved as an object.
The layer xRef Object Lab .HIDE is provided for this purpose. This layer is always hidden except for the layer combination 'xObject Lab'. Keep the slab there, so it's out of the way, but you can modify it if necessary.
1. Draw the Shape.
Use any 2D tools, but finish by making a fill. (Good time for the Magic Wand.) You might do this in an elevation window. Copy and Paste the shape into the Plan window. Move the shape to the Global Origin. (You know where it is, right?) Move it to the Object Lab layer, and activate the Object Lab LC.
2. Make the Slab.
Set up the slab tool with the thickness you want for your bracket. Use the Object Lab layer. Magic-Wand a slab onto the fill.
3. Set the 3D Projection.
In the 3D Projection Settings, set the view to elevation, and roll the camera around to the top. This gets you looking edge-on at what will be the top of the object.
4. Get the slab in the 3D window.
Select the slab and go to the 3D window. Select the slab.
5. Save As an Object.
In AC7: File | Save As... File Format: Object.
In AC8: File | GDL Objects | Save 3D Model As...
Save the object in the appropriate project library. Check the box 'Remove Redundant Lines from Symbol'. Select 'Editable GDL Script.' Click Save.
6. Place the object.
Return to the Plan window. Activate the object tool and place the object; it will be the default.
In order to have the pen and material in the settings take effect, you must uncheck the 'Use Object's Materials' and 'Use Symbol Colors' boxes in the settings. Alternately, you can go into the scripts and replace all the pen and material references, but that's another post.