So I opened the .step file in FreeCAD, selected the “battery cover” component, exported to 3mf, opened that in Creality Studio and printed it.
Looks really nice and feels nice on touch as well 
(except that my desk and the phone itself look terribly dusty in the pictures). It’s plain “hyper-PLA” filament on a 0.4mm Creality Hi printer.
Next time I’ll put some embossed logo or something (my printer supports up to four filaments for multi-colour prints)