£55 from Freeborn. Not up to the task? Depends what you take the task to be. If it's to protect the battery and frame by absorbing the energy of a rock flung up, albeit by sacrificing itself, then I reckon it is up to the task. Having said that £55 is a heck of a price for a plastic moulding (but a lot less than a battery or frame repair).
£55 from Berkshire Cycles. They’re happy to post them out. Smashed mine last week when a rock was flung up by the front tyre. But repaired rather than replaced.