Nano 6th Gen moisture issue and case advice needed

I bought a 6th gen Nano 16gb to use while I run. It completely died after 6 weeks. It was replaced with no explanation at my local Mac store. Six weeks later the replacement died and I was told that it was moisture damage. I never used it in the rain or spilled anything on it. It seems that merely clipping it to the collar of my sweaty shirt was enough to kill it. They replaced it again with a warning that this was the last time. Yesterday I went running in 40 degree weather (minimum sweating) and wore a second shirt over my regular shirt to have a dry place to clip my Nano. 40 minutes in, my unit died again. grrrrr

I called and they agreed to replace the unit again and strongly suggested that I buy a protective case. Do any of you guys use a 6th gen to run with and what kind of case has proven to be effective for moisture isolation?

thanks

Mark