well this forum doesnt like you posting links to other poweriser sites but there is a tutorial that shows you how to do it here:
http://www.geocities.com/powerisertiresocks/Lots of people have done this and some have trimmed the tyre more than others but it seems like a lot of work to end up with something that doesnt look too great and the official part is 15 quid!
I dont think its fair for any single person to say the rubber soles last a few months or whatever since people use them differently (some use them on tarmac, concrete, grass, indoors etc).
If you're going to go with the tyre thing then I suggest buying a new tyre from halfords and I wouldnt be too worried about grip as everyone says its fine, even if you use a flat-style gripped tyre (known as a road tread, rather than a mountian bike style tread). It seems perfectly safe, but you have to attach them tightly, neatly and test they dont slip off or move before normal use.
