^ Dan - This is exactly what I thought about mate. It's the obvious mounting point as both my amp and sub will fit and means I still have full access to the boot as it is out the factory
If it works, then it's a tidy fit. I don't think they'll be any heating issues as I'm honest. Previously the amp was mounted on the back of my pre-built sub-box and if mounted on the rear passenger side seat, it will be in the same position in essence with nothing enclosed to make it overheat.
I had some light mesh stuff from a material boutique before which covered my speakers on my old Punto's parcel shelf. Something similar again do the trick.
My only concern is on the rear seats, is there enough MDF wood for the depth of the screw? I don't want the screw to plow into the upholstery of the seat.