Well, as a consolation prize, I can imagine setting one up so that it isn't much larger than the servo itself.
It's pretty easy to get 3 terminal snap switches with arms/rollers on them. Take one of the round servo control arm disks (no idea what they're called), clip off most of the disk, leaving > 180 deg. in place. Mount two of the snap switches to a servo, one on each side with the arms extending up so that they will be contacted if the disk touches them. In the neutral position, both switches should be pushed down, if the disk rotates a little bit in either direction, one of the switches will be released.
Next, wire both NO contacts to the + of the battery, the NC contacts to the - of the battery, and wire the motor between the two remaining terminals.
good luck