Fortunately, this problem was solved by 3M for their 6200 series respirators several decades ago, and their patents on the design have expired. We have reverse engineered the design for 3D printing.
In order to fit the print on a 200mm build plate, the strap was broken into 2 parts that snap together. It is less than 1mm thick, and it may be printed in rigid material like PLA without support. If there is stringing that interferes with the buckle insertion, it can be cleared out easily with a small screw driver.
To further optimize the mask support, we recommend using 1/2 inch elastic and crisscrossing the straps such that the printed part (that sits on the crown of the head) attaches near the chin of the mask, and the strap around the back of the neck attaches near the nose of the mask. This results in minimum force requirement from the elastics as the force vectors will be applied normal (perpendicular) to the chin and the nose surfaces for optimal sealing against the face.
Progress on the design can be tracked at https://bemask.org/.
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