I do like both of your ideas James, even tried the front speaker one, damn magnet was too big and I couldnt bring myself to break out the hammer to do the necessary!
The rear though, ive got rubber washers (kind of) but I need to get some of those speaker grilles that you have to protect them, I get them and im away! THEN I might even ask you to build me a small sub!!
On a slightly unrelated note, im also after a facia for my stereo as the one I found simply doesnt fit, lugs are in the wrong place (black rectangular piece of plastic that fits on the front to tidy it up)