My guess from your post is that your boat walks to port in reverse. My boat does too. To minimize the walk, we've found a strong burst of reverse with the rudder hard over to counteract the walk tends to work. As the rudder grabs and the boat speed in reverse increases, you may need to straighten the rudder a bit so you don't go too hard to starboard. Then, with the boat still moving aft, switch to forward and quick push the rudder over to the other peg. This will tend to rotate the boat in place if you do it exactly right. YMMV. Recommend you try this maneuver away from obstructions first, and learn what your particular prop/bottom configuration likes to do. Good luck, full keels and in our cases "skeg" (this spell check is a pain) hung rudders can do some interesting things re: prop walk.