Mick -
 
  I use a piece of welcome mat cut to size to cover the floor of my
critter's cage.  It's essentially outdoor carpeting with a rubber
binder on the back; the mats are available at home improvement stores
(eg, Hechingers or Home Depot) in sizes up to 3'x5' for around $10.
 
  A razor knife will cut this stuff very easily from the rubber side;
from the carpet side, well, my guys have been clawing at it for half a
year now w/o significant damage.  The rubber backing keeps the carpet
from unraveling when you clean it (though it's too big to go in the
washing machine; I take it outside and hose it off).
 
  If the piece is cut to exactly fit the metal cage pan, the lower
edge of the cage will hold the four sides of the carpet down very
securely (so they can't go exploring underneath...)
[Posted in FML issue 1140]