Floating shelves can start as any other cabinet. For example, we can place the wall cabinet "Open Shelves" from the Frameless Library. Then in the cabinet editor, click on the blue "Parts to Build" button and click "clear all" to uncheck all of the parts. You should then be left with only the interior (the shelves) which you can adjust as normal.
Next, go to the shape tab and click on the back, left, and right sides changing their type to "nothing". This will allow the shelves to exist by themselves so that their measurements won't account for any other material dimensions.