This is a design I made to keep the straps down on my 3M respirator. Without it, the straps would stick out diagonally and I felt goofy. I used Happy 3D to make a tapered rectangle 40mm x 18mm x 6mm, used half circle shapes to create the space for the holes, then added elongated cylinders to accommodate the 13mm wide straps. You can take the design and stretch it if you have a similar mask with wider straps. 

To print, I rotated it upwards like a brick to avoid having to print supports and it came out best. I set my temps at 200 for the extruder, 50 for the bed, and kept the back fans on the whole time for PLA filament.