In many cases a pair of heavy kitchen sheers will cut the thinner plastics, and a quick run over with a flame (lighter) will melt the edge slightly so it isn't too sharp.
You might also be able to cut holes with an old butter knife heater on the stove and 'cut/melt" the door into the plastic if it is too heavy for shears. Garden pruners can work too (if you borrow them don't tell what you want to use them for the owner might not loan them to you... or to me...)