Ah, My favorite topic!
First off, rip out all the eFX padding (even the stuff around the eyes) they try properly padding it. That may change things a bit. (Unless you're trying to keep it in a collectible condition, but then you shouldn't be trying it on for trooping! )
Now the CAP-W and eFX are shaped a bit differently, and I believe the CAP-Ws are a bit more circular, but in general the eFX is larger than the CAP-W. So if you grab a Cappy then it'll be a gamble on whether or not you'll fit in it.
I've heard RT-MODs are larger and they look nice, however I do not own one.... yet. The other option is an FX/AM bucket, which are the largest ones you can get (That I know of), however accuracy in the small details are lacking. People will still know what you are at a glance though. (If you post something up in the junkyard, there may be some trooper with an FX/AM helmet that he or she wouldd be willing to part with.)
-iz