Door opens, wind hits door, nose goes right, said wind deflected up to bottom of wing causing small positive roll force.
At the same time, open door masks airflow over the vertical stab, so the angle of attack put on the fin will not be as effective in getting rid of the yaw condition.
Dont forget about the prop wash. The door is right in line with it so an open door will have a significant amount of extra high energy air to play with. What flight attitude were you in when you opened the door?
Is the airframe "straight", ie will it fly normally without any turning tendencies when trimmed?
Most small Cessnas require some rudder input even during cruise flight to avoid sideslip.
What is the fixed trim tab on your rudder doing for the aircraft in level flight?
if it is preventing the A/C from yawing to the left in the cruise(to relieve your aching foot), then masking the rudder would alter the effectiveness of the device, possibly leading to a yaw back to the left.
Finally the keel effect of the fuselage being thrust sideways into the wind is probably going to force the tail to the right and the nose left, at which time the door wind deflection force will be additive and help to continue the nose to the left.
The left wing retreats slightly and drops, the nose falls further into the slip ...
By the way, do you end up in a spiral dive if you let it continue?
Cessna 172's had their flap 40 setting placarded years ago. No slips with flap or something similar.
Masking the vertical stb/rudder is not something Mr Cessna wants you to do.
Just a few idle thoughts, and i'm not an aerodynamicist or even a trick cyclist.