We all know Apple is fond of glue, but now you have spot welds to deal with! Underneath the cover is an adhesive coated aluminium plate, welded to the aluminium body in no less than 100 separate locations.
This is where your drill comes in. Drill out each and every weld, being careful not to drill too deep or damage the externally visible edges.
Your drill holes should not be deep enough to have straight sides, so they shouldn't really be holes at all. Just a countersinking depth will do.