Add triangle at inside 90 degree corner.  Add a beveled fillet to an inside corner.

Version 1

    Here is the SVRF to build an inside corner 90 degree triangle with equal sides.

     

    VARIABLE minInsideCorner 0.5 // minimum length for inside corners to add triangle to

    a = CONVEX EDGE <starting layer> ANGLE1 == 270 ANGLE2 > 0

    //Warning: both edges must be the same length to create a triangle with a 45 degree hypotenuse.
    b = LENGTH a >= minInsideCorner

    Triangle_Inside_Corner = EXTERNAL b <= minInsideCorner ABUT PERPENDICULAR ONLY REGION

     

    Right triangle added to a 90 degree inside corner