Do you know that a metal detector starts to break down in a backpack already? An armrest and a screen are the first to suffer. If you go for a hunt by car, a hard case is the best option to transport your metal detector. See the photo.
There are special cases designed just for metal detectors. But you can use universal ones as well that can be made over inside to fit the right shape of your machine. Also, for your detector you can apply hard cases for large musical instruments.
