A variation on the CD fixture produces a 3.5 inch hard drive platter fixture:
Which needed just a touch of milling for a snug fit around the platter:
The rather faint line shows engraving at -1.0 mm = 70 g downforce isn’t quite enough. Another test with the same pattern at -3.0 mm = 140 g came out better:
It’s in the same OpenSCAD file as the CD fixture, in the unlikely event you need one.