Cool helm!
As far as crests go, we have helms that actually do have holes in them specifically for mounting crests, so it's not that bad. I've got two tiny holes on the top of my helm, and that's it. My entire crest is removable, though it would be a bit of work.
Anyway, my crest is solid wood. I don't really feel the weight, and they are super sturdy. You won't see any other knights lugging about their helms by the crest!
As to historicity, a Teutonic Knight could receive gifts with the permission of his superior. So a helm with a crest could be a gift from your secular family, and you could have it with your Komptur's approval.
Or you could be a high officer, though portraying that gets really really hard with accouterments and encampment.