Help with square corners on wet molded pouch

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I’m making a wet molded pouch that is more or less square. I’ve got the form and the leather is in the shape I want but the corners are wonky. I’ve tried to search you tube but the videos I’ve found are for flat corners or gussets. There should be a way to make a cut that will result in a more square corner but I can’t figure it out. Project is the mid pouch on the back outside of a scout carry sheath where you would carry altoids tin or brass tinder box etc.  Thanks to anyone who can assist. 
 

Bigs