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I have a posture correcter that I try to wear a few hours a day to help with this. Highly recommend. This is the one I have (link to Amazon). This particular option is out of stock until April 12, but there are other brands out there too.
I don't know the character well, but it's a holster of some sort right? To carry a sword or staff on his back perhaps?
In any case, you can easily make that thing using vinyl or pleather from a fabric store. OR you can get one of these posture correcting things from Amazon.