Not awesome looking upgrade parts but they'll do the job of staying longer and protecting your gearbox. These are reinforced anti-heat and anti-wear plastics and can fit existing G3 and MP5 AEG gearboxes up to 8mm in specifications. Both cost the same at Get Reload, where you can get them for US$8.50 which is a price worth putting up for if you want improved selector plates.

Lonex Anti-Heat Selector Plate For G3 AEG Series

Lonex Anti-Heat Selector Plate For MP5 AEG Series