easy to do and easily knocked when putting things in the boot.... Bolt it straight to the back of the seat

Harder to do, but more professional.... Make a small bracket which would mount the changer under the parcel shelf support. It would be held up out of the way so wouldn't get knocked or damage if you put a lot of things in the boot.
If doing a boot install.... making up a MDF panel which would cover all the suspension turret. A changer could be mounted behind it with the front panel poking though. Or cut a door into the panel which hides a Changer, amp, sat nav, etc.