I'd use an angle grinder with a thin cutoff wheel and basically just cut either the locking nut or the whole cylinder in half. If you're reasonably careful, you should be able to do this and barely touch the fiberglass (if at all). You're much more likely to cause cracks or other damage if you are putting torque on something, and it's much easier to have fine control over an angle grinder than a reciprocating saw or a drill.