Add text-alignment, box style, and a GUI receive to my_canvas. Take away the "selectable size" and make the entire rectangle a clickable message box.
Default box style would look like a normal message box, except it has a second inlet for setting gui-properties (hence the extra receive). Dimensions, border width, bg color, text properties (alignment/size/font/etc.), would all be settable through the properties.
GUI style would be a rectangle, like my_canvas, but with inlets/outlet, and border.
Inactive style would take away the inlets and outlets and make the object non-clickable in noneditmode (similar to cyclone comments, but with a (possible) bg, border, and alignment).
dollar sign expansion might be confusing to the beginner, but it would work the same as it does currently: anything typed in the message box gets expanded at "message time", and "properties" in the GUI settings work as they do for all GUI objects. Plus there might need to be an option to lengthen the rectangle as needed for variable width fonts (or constrain it to a certain size).
This would make formatting text in help files much easier, and make message boxes more flexible and gop-able.