Questions about working with round_corners() & arcs

[dan-p3rry]

Two questions about the following code:

1. I want to round the corners of a 120 degree arc. Unfortunately round_corners() doesn't differentiate between the $fa segments of the arc and a sharp corner, so I cannot round the sharp corners with a value of r larger than $fa. Question: is there an alternate way to round the corners? If I was working with a 90 degree arc, I could let round_corners create the arc from a rect(), but I don't what the starting shape might be for a 120 degree wedge.

2. In the code below, I use offset_sweep to turn the arc into a nicely rounded wedge of cheese. Then I need to open a window in the rounded face of that wedge of cheese. If I wanted square corners I could use rotate_sweep(). I spent a lot of time in the documentation, but couldn't figure out how to open a round-corner window except the hull() of four cylinders. I'm kind of lucky even this worked -- depending on what additional shapes are present, this may not have been an acceptable solution. Question: can you think of another way to open the window? Here's a picture:
   

Thanks & happy holidays!
Dan

`
include <BOSL2/std.scad>
include <BOSL2/rounding.scad>

$fs = 0.4; $fa = 3;

diam = 151.3;
shell_th = 3.6;
axispt = [shell_th/tan(60), shell_th];

container_shell_120(80);

module container_shell_120(height) {
rounded_arc = round_corners(r=3, arc(r=0.5*diam, angle=120, wedge=true));
interior = offset(rounded_arc, delta=shell_th);

difference() {
    up(0.5*height) 
        offset_sweep(rounded_arc, height=height, bottom=os_teardrop(r=3), 
            top=os_circle(r=2), anchor="origin");
    up(0.5*height + 2)
        offset_sweep(interior, height=height - 2, 
                bottom=os_circle(r=2), top=os_circle(r=-3), anchor="origin");

    #hull_arc(height);
}

}

module hull_arc(height) {
// angle of sweep = 120 - 2angle corresponding to 10mm of arc
sweep_angle = 120 - 220360/(PIdiam); // 15 is 10mm of arc + 5mm for corner rad
hull() {
up(20) zrot(60 - 0.5sweep_angle) right(0.5diam - 5)
xcyl(r=10, h=70, anchor=LEFT);
up(height - 20) zrot(60 - 0.5sweep_angle) right(0.5diam - 5)
xcyl(r=10, h=70, anchor=LEFT);
up(20) zrot(60 + 0.5sweep_angle) right(0.5diam - 5)
xcyl(r=10, h=70, anchor=LEFT);
up(height - 20) zrot(60 + 0.5sweep_angle) right(0.5diam - 5)
xcyl(r=10, h=70, anchor=LEFT);
}
}
`