That would not be improper in and of itself. The improper use of a sacramental is that which shows disrespect toward the people they signify, or that which lends to superstition.
By medicine bag I assume you mean a leather pouch worn around the neck as opposed to the larger bag a medicine man might have worn over their shoulder.
It would really depend on why you did so. I am not a native american, but I had a friend in high school that was. He used to keep all sorts of items he considered magical talismans in a leather pouch around his neck. He said his grandfather taught him to use them for vision quests and connecting to his brothers and sister in nature. Not sure how much was authentic spirituality as apposed to just being different.
If you are keeping them in a medicine bag as a sort of talisman then it might be inappropriate since sacramentals are not magical items. It also can lead to weird intermingling of different spiritualities. On the otherhand if you simply don’t like chain necklaces and want to to keep them close then I don’t see an issue. If it’s the later I would refrain from calling it a medicine bag though since it has spirituality overtones. I’d simply refer to it as a leather neck pouch.
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