Bag experts: Square, rigid corners

[NJP]

Greetings

I need to build a bag with square corners and hidden seams. 

I started with 2mm veg tan, stitching a gusset to front and back panels and then turning the flesh side in... But my corners were "puffy" and the gusset curved out at the top. 

The attached images are of a handmade bag built by a pro. Did he use some heavy internal stiffener, or can this be achieved with leather and stitching alone? 

Notice the square corners at top and bottom of the gusset... That's what I'm after.

Any suggestions would be greatly appreciated.