bonj said:
you don't buy carbon 'cos it looks nice, you buy it 'cos it's light. Get a (black) laggy band on it. Or just don't bother with cadence
Spot on, get better at estimating your cadence and that means there's also less to go wrong.
If you're carbon frame was to fail I know most manufacturers include some sort of fail safe guarentee. They might refuse replacement.
Have you considered a shimano flight deck? They sort out your cadence without any magnets. You do need to know your cassette sprocket sizes though.