i don't have Eczema, but do get a rash or chaffed on hot humid days,

I grew up on Ammen's Medicated powder,

damn good stuff when you need it

Not only do I still have some Talcum Powder in my medicine cabinet, I have been known to.... Spray a weed with RoundUp!

 
I used to keep a small bottle of it in my shooting bag, when I shot competitively. I dusted my hands with it in between stages, helps keep them dry.
 
For the good stuff, look for talc from a Billiard supply company. That is typically pure, unscented talc. The bottle I have (got from Amazon, IIRC), is Silver Cup Premium Talc.