I took apart a broken servo, and tried to remove the potentiometer, but it was a part that held the plane together, and had to stay in. A standard, or even 12 gram servo placed on the bottom bellow the door can close it, but would get damaged if cargo slipped back when it was closed, so the 9 gram in the top would still be needed. If set to a two position switch it can close the door after it is opened. To keep it from being stressed as the cargo falls out the door can be braced at the fully open position. The on problem is that it requires a 7+channel radio for all the controls. Differential thrust or the rudder servo can be sacrificed for the last channel. If you have the time pinning the rudder is probably the better option because Differential thrust is more than enough to turn it. Just be prepared to land it without rudder if your mixing isn't good.