Great question — answering as the UphillTech team:
I would try a couple things: Tap a tool like a small flat-blade screwdriver or awl in from the end to try and separate the metal at several points. Put a little heat on it. Hit it with a hair dryer until it's just hot to the touch. It might expand and soften a bit. Tap on the metal from the end to try and jar it loose. Use an abrasive wheel in a rotary tool (Dremel) to carefully score the metal lengthwise so it can be split as in step 1.
Full answer by isherwood: https://diy.stackexchange.com/a/262138 (CC BY-SA 4.0).
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