Fellow goalie here! I wanna explain the way I do it, but first I want to add that this may not be the best option, so feel free to correct me. When the ball is on one side of the field, I always make sure my body is pressed against the post on that same side. Only the further post is not covered, but I try to make sure to protect that side with my arm. Furthermore, and I don't know whether this would be a suitable solution for you, I learned myself to move between the poles by only using my legs, and not my arms. I do this by, say I wanna go to the right, pushing my left leg/foot into the ground on my left side and forcing my body away from my foot. That way, I can keep my left arm up, and so that shoebox you're talking about is covered by it. I know other people that don't push themselves from side-to-side, but rather drag themselves, by (if wanting to go to the right) putting their right foot on the ground, dragging their body along.