You will find all knife of holster making tips and answers to just about any leather working questions at leatherworking.net...
1/2 the width of the gun plus the thickness of the leather
I make leather IWB holsters for the G26 if you're interested. They do run a little more than $50 however. Something like this one is $80...
I've been making custom holsters for 15 years and can make what you're looking for.
S&W M&P9c OWB1 with thumb break
I have a mold for the PM9 and can make what you want.
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