GTDers in the UK will probably be aware that magnetic book ends are about as easy to get hold of as unicorn tears.
Instead I bought some of these: http://www.amazon.co.uk/gp/product/B000J69E5M and stuck on some magnetic tape. the magnetic tape made little difference since they come with rubber pads on the bottom anyway.
I now use a pair of these in each cabinet drawer (I tried one each and the back folders needed more support), bent about 30 degrees to make folders easier to read and they seem to be pretty good. They seem to self adjust when folders become more and more full.