The groove in the fork tube, is not 'square shouldered', it's just a semi circle, I would be hesitant of putting a snap ring in there, it may come out at a very inopportune time .
I devised a method to depress the top plug, I take a piece of inner tube, wrap it around the fork tube, and then put a small three jaw puller around the outside of the tube, and then place a worm gear hose clamp around the jaws of the puller and tighten it up , then used the threaded screw of the puller, to press down on the plug and push it down out of the way, to gain access to the circlip .