I would do it a bit different Wotto

Instead of the plastic going into the wireform, I would have it stop at the wireform & mount it using the existing holes for the ball guide mount screws, one is above and one below the flipper. You may need to cable tie the plastic to the wireform for added stability - this wouldn't be too bad as there are other plastics already cable tied in the game

Hard to explain without a picture but my plastic would be the same as yours except the long edge would be cut back about 1cm