if you look in your local phone book you should be able to find either a
builders salvage yard or a discount builders supply where you should be
able to find a cheap door to fit your closet.

put the original door under the bed or in the basement and make your
cuts in the new door you bought, then, when you move, you just put
the original door back.

or you can jury rig something like this, which is nothing more than
cardboard nailed in place and held shut with velcro.

you'll note that my corrugated 'door' doesn't fit very well so it in no way
seals my veg room, but, it sealed it well enough to bring my temps down
from high 80's to low 70's so definitely good enough.

anyway, i hope this helps.