For a Graphic, the border-style option is probably easiest.

{Canvas border-style = "double", border-width = 3px}

Option borders are singular for a Graphic. BorderSpec just controls each side's border width and margin width separately.

You could certainly override the draw method to add any such decoration, or anything else for that matter, though you'll want to ensure that you leave space for that decoration -- shouldn't be difficult with a Canvas. As a simple, but somewhat garish, example:

{import * from CURL.GUI.CONTROL-HELPERS}
 
{define-class public open BorderedCanvas {inherits Canvas}
 
  {constructor public {default ...}
    {construct-super ...}
  }
 
  {method public open {draw renderer2d:Renderer2d}:void
    {super.draw renderer2d}
    {self.overdraw-border renderer2d}
  }
 
  {method public open {overdraw-border renderer2d:Renderer2d}:void
    def bounds = {self.get-bounds}
    def left = -bounds.lextent
    def right = bounds.rextent
    def top = -bounds.ascent
    def bottom = bounds.descent
    def psize = renderer2d.pixel-size
    def psize2 = psize * 2
    {draw-standard-bevel-edges
        left, top, right, bottom,
        "black", "black", "red", "red",
        renderer2d
    }
    {draw-standard-bevel-edges
        left + psize2, top + psize2, right - psize2, bottom - psize2,
        "green", "green", "blue", "blue",
        renderer2d
    }
  }
  
}
 
{BorderedCanvas background = "pink"}

I would certainly prefer using a Canvas inside a Frame (or even, if you are using styled controls, a SkinnableFrame, for more options).