Elevator trim
From Bearhawk N813Z
Addition of a simple bracket to which to mount the electric trim servo on the underside of the elevator. Composite blister style fairing will cover the recessed servo. A push rod will emerge through a slot in the fairing and attach to the control horn on the trim tab.
ECU for trim
Per @akschu, using the Rugged MEGA-ET. This is an extended temp range model with better more rugged specs than the Arduino equivalent.
Bristol SG servo integrated position sensor can be read by its I/O pins.
For driving the servo motors with the MEGA, a motor driver sub board is required: https://www.pololu.com/product/2519