I designed this for the Giant D Fuse seatpost - 3D printed in black PETG. It attaches with the same bands that the Garmin mount does. If someone has good dimensions for the Specialized post, I can work on a design for it. No promises on a quick turnaround, though. I plan to be in a turkey coma for the next day or three.
If there is interest in the D Fuse design, I’ll throw it up on Thingiverse.
(edited to add Thingiverse link)