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Are you looking for a square brush, or just something that simulates a flat brush?
I haven't found a great way to simulate a flat, because even at 1% spacing, you'll see gaps on any kind of change of direction. Since Photoshop brushes are just stamps of shapes repeated very close together, it's inevitable that some shapes don't lend themselves well to being brushes and getting what you expect out of them.
For short strokes that only go in one direction you can make a more rectangular shape and set it to 1% spacing. In Shape Dynamics, turn on Angle Jitter and set it to Direction. Any line that changes direction by too much though will end up with annoying gaps as the stamp tries to navigate the change. I like to round the edges of my rectangular shape to make the lines seem more smooth.
Better start saving then since I've used this software to make the brush strokes, Ps will always break the brush stroke when using flat brushes and since flat brushes are very import to me I pretty much block all my colors and shapes using that flat brush I showed, plust you have two types of blending options!