Join Date: Jun 2002
Location: Brussels, Belgium
Car: MG TF
Here is how I did it:
I went to a "classic car" parts shop and bought a big piece (1m x 2m) of thick and heavy sound insulation carpet (about 2,5 to 3cm thick). This is the same sort of thing as the thick carpet found under the black upper carpet in the hood well. I can't remember the price, but I remember that I found it surprisingly cheap.
Then I pulled up the black carpet in both footwells, just enough to get access, and I used a big piece of cardboard to make a precise "model" of the shape of the footwell. I used that "model" to cut two bits out of my new insulation carpet (left and right footwells are mirror images of each other).
I then put both these new insulation carpets on top of the existing ones and put back the black carpet on top, and... Bingo!! The edge of the carpet was now firmly seated in its slot in the door seal! I had to do a tiny bit of trimming of the black carpet with some scisors around the front/speaker part of it, but after that the fit was perfect and it hasn't moved 1mm in the three years since I did it.
Additional "collateral" advantages of this solution are that road noise insulation is improved and the driving position feels somewhat lower since your legs rest a few centimeters higher than before.
I can really recommend this method as a definitive solution to this problem!