Physics matters: aluminum doesn’t magnetize. So that’s not it. ( you can test this easily enough. Take a kitchen magnet and see if it will stick to the door frame. If it doesn’t, the material is not magnetic and cannot interfere with the sensor in the way described in the gate topic.)
http://directmetalsrecycling.com/steel-vs-aluminum-how-can-you-tell-them-apart/
Aluminum can still however block signal.
Zigbee repeaters do not boost the signal, they just pass it along. So if you’re putting the mains powered device inside the same out building, it’s not likely going to help. You won’t be able to get the signal to it either. Repeaters are used to extend the distance. But they can’t increase the signal’s ability to move through a blocking material.
Try the following topic (this is a clickable link). It discusses most of the issues you run into with outbuildings.
And this one discusses some additional options. Your idea of rubber seals can work very well. They tend not to block signal but they do block cold air. Mostly you’re just trying to find a way to create a path for the signal from one additional options.