What I did was cut a piece of MDF to the size and shape of the amplifier. This was installed and secured under the carpet (you can use double sided carpe tape of very strong velcro).
Then place the amp on-top of the carpet, and secure (with short screws) through the carpet into the MDF.
The thing will never move because a) the MDF is secured to the boot floor and b) in order for the amplifier to move, the carpet has to - as the carpet doesn't move, the amp never will!
Like I say, I've never done this on a Mk2, so just make sure that the seat can move backwards and forwards without fouling the amp, and that there's plenty of room for air to circulate around the amp. Be careful choosing your poistion, as you don't want rear passengers kicking the fcuk out of it when they sit in the back!
I'm sure if I can fit one safely under the seat of a scoob (where the seats are one the deck!), you'll be fine with the Mk2