I am no expert,but I think letting it dry a bit more may help.And also it may be helpful to crease the fold,so the bend may be more concentrated in that area?Like I said I am no expert,and I have been putting off some sheaths of my own,so I haven't done any in a while.I will be the first to tell you my advice might be worth what you paid for it.
Just trying to throw some Ideas at you and hopefully help.
Robert