Slick, a good airfoil that works at model size and speed for the Extra and that would be in keeping with the scale airfoil shape would be the Eppler 474. It has the same sort of shape but at model speeds would be a touch more stall resistant than the blunt nose/flat rear shape of the full size Extra.
Some years back some local buddies were looking for a more stall resistant option for their models for Control LIne Fast Combat. The popular design at the time used a flat rear section very much like the Extra that was designed by Phil Granderson. It did OK but had a nasty habit of suddenly stalling if the loop/turn size was tight and the model slowed down too much from consecutive maneuvers and reversals. I suggested the E474 and the guys built a half dozen using that airfoil. They were immediately sold on it and used it to good effect over the next couple of years. It allowed the model to either turn tighter or to turn the same but not bleed as much speed. And when it did stall it more or less just wobbled a bit on the lines instead of coming in at them. And that's a good thing when you're looking at a 125 mph skill saw heading your way...
In any event if you were to use the E474 on an Extra model I think you'd be happy. If it were to have a problem it would likely be that it resists stalling so well that it won't snap cleanly on demand if the wing loading is too light. But it's easier to add stall strips to the wing to encourage a stall than it is to correct an overweight plane with a stall prone wing that wants to snap and spin.