As an alternative to an enclosed unit, how do they compare and is fitting to the rear parcel shelf a viable option?

As long as the sub is designed to work in an infinite baffle/free air, it'll be a definite step up from no sub :)

If you're thinking of this, I'd honestly try to listen to Genesis 6x9 subs...

OK, cheers Nick. I am exploring all the options so I'll have a look at those next.