I have several servos mounted the way you described. I have not noticed the arms being to short. I do however use the longest 4 sided arm that comes with the servo. Maybe your mounting the servo to low. I do mine as follows. I make the servo cover out of ply. Then use two hardwood blocks to mount the servo on the hatch. I have the side of the servo case right up against the hatch. Then I mount the hatch to the wing again using hardwood blocks and servo screws.

David