This kind gent seemed to know a thing or two about the 4iiii PRECISION power meter. Multi-channel, super accurate >99%, left/right/both options, 200 hours battery and US$400+sales tax (GBP TBC probably 350-400 inc VAT). Is it any good though? (It’s glued to the inside of a flat non-carbon crank)

99% accuracy sounds pretty good to me!