Some people make the automail out of craft foam over a glove. It looks REALLY spiffy. Time-consuming, but SPIFFY TO THE MAX. But it won't necessarily be cooler (meaning temperature). If you are trying to make your Ed costume less hot I would suggest getting a jacket that isn't a sweater
This is probably way easier than making automail.
"Tell him 'Fine, there's no way I'm dying before you do,
you morally bankrupt colonel with a god-complex.'" - Edward Elric
Going to ALA, Gallifrey One, and SPN Vegas Con