I use one of those inexpensive plastic/soft vinyl printer covers (@$5-10 on e-bay or Staples), but you have to make sure that you have a stylus guard so you don't inadvertantly catch it and bend the cantilever. If your cartridge doesn't have a dedicated guard I've found a cut down paper towel center (the cardboard tube) make a nice guard. It isn't pretty but it works.
Alternately you could just put an old scratched record on the platter when not in use, but that doesn'c cover the wholt table, only the platter.
good luck,
Bob