You could drop the spare tyre for the gunk in a can and a 12v pump.....so many cars come with them these day. Just make sure your tyres are all in good health
My packing tip is to have more bags to put your clothes in. that way its easier to shove them in the corners and mould them to the boot space over a couple of bags rammed full of stuff.
Dont forget you have a bit more storage behind the passenger seat
If you can get an adapter for your 12v pump (if it has a suck action)........ maybe you could use those vacuum bags http://www.wilkinsonplus.com/Clothing-Storage/Russel-Storage-Vacuum-Bags-x-2/invt/0232360