Vex makes a plastic arm, and so does marcum...If you have a Humminbird you get the marcum arm some washers and a screw to mount on the huminbird pack.
The arms on the vex and marcum's are fine and don't break very easy in the cold.
One of the reasons for the custom built arms was due to the arm on the showdown....the original arm with the showdown was a very brittle plastic that snapped easy when it was really cold out. So people started buying a $30 aluminum custom arm replacement to fix the issue vs buying a dozen plastic ones....I think they might have fixed the plastic showdown arm issue though.
I wish we'd have a guy on the site that would do custom stuff like transducer arms & drill/auger/power head adaptors.