Take out the carpet from the boot floor, measure it and nip down to B&Q to get a big enough sheet of MDF.
Lay the carpet on top of the MDF, draw around it and use it as a template to cut the MDF to the right shape. Test it to make sure it fits and you can actually get it in and out of the car.
Get yourself some spray glue and some carpet to cover the wood in. Glue the carpet to the MDF floor you have just made and make sure it looks tidy. Autoleads do some carpet that is a very similar colour to the standard stuff, so nice and subtle looking.
Once done, pop it all into the boot. Leave the original carpet in the boot and place this on top though, as this helps to stop the MDF floor from knocking and banging around. When it's all in there, remove the subs from there box (I'll explain in a minute...) and put the sub box where ever you want it, making sure it doesn't foul the rear seats closing or the parcel shelf.
Once you're happy with the location, screw the box to the newly made floor from the inside of the box. One screw in each corner should be enough.
Now put your subs back in the box, stand back and admire your handy work
It's only a basic false floor, but it does the job! You can use this later on if you decide you want to go one step further and turn it into a box to house your amps and other bits and bobs.