What if your whole portal line was the checkmark ? Green if selected and grey if not ?
On clicking on a portal line you add its pk to the $$var, on another click you replace its pk in $$var with "".
Just put the pk in the portal, make it as large and tall as a portal line, set a padding big enough to make the text disappear and use conditional formatting to color it.
Remember Fitts's law.
To get a uniform look, I would not try to match the check box formatting on a check box field. I'd see about replacing the check box format with a button with hide object when used to make a "check mark" appear and disappear. Clicking the button performs a script that sets the needed value.
That should then make it possible to get the same "look" in both cases.