[MovedGoalPosts]

I'd get a bit of tin foil or something. By the time your spent effort locating and buying blanking plates, you could have bought a new cheap case.

Some ventilation into a case, not fan assisted is good, and indeed essential if your case fan(s) are only extracting - the cool air has to get in somehow.

If dust really is an option look at where the computer is sited. Try and keep it out of dusty floor mounted corners of rooms. Periodically most computer cases need a good clean out of the accumulated dust, whatever standard the case.