Mount this box to your Barn Door tracker for clean installation of an Arduino Nano and the stepper driver.
Print the lid on its top to avoid using supports, then simply slide the lid onto the dovetails. You'll need to drill pilot holes in whatever location you choose to mount it, but be sure to leave clearance for your tripod mounts.
Optionally, add a small push-button switch (pictured,) and install this remixed firmware to provide on/off and reversing functionality:
Click once = On
Double click = Reverse Direction
Long press = Off
Important! You'll need to include the CheapStepper and OneButton libraries in your Arduino 'libraries' folder.
Why would I reverse direction, you ask? Perhaps there's an obstruction from your view point, like a railing or cliff, and you can only access the tracker from the north side. Or maybe, the position of your camera is throwing off the balance of the barn-door as it rises, so it'd be better for it to lower instead. It might be a seldom-used function, but I thought it might be useful...plus, it's COOL!
The remixed firmware also provides parametric control of the steps per revolution since there are a couple variations of these motors. To adjust this, simply comment/uncomment rows 23 and 24 in the .ino file, (the file is heavily commented in case you're unclear what to change.) In addition, this firmware is adjusted for 1.09 turns of the big gear per minute (based on the comments in the 28BYJ-48 remix.)
Bonus! Use my calibration dials to determine how many steps per revolution your particular motors use. https://www.thingiverse.com/thing:2007795