Thinking of the 900d, If you set the case up with a pressure optimized fan setup you should be in taking more air from the top and front and sides than is being exhausted out the back. Which means all the in takes will be filtered and there will be lest dust coming in from the back as it will be pushing air + dust out.
So shouldn't be too much of a problem, you can get dust filters for the rear fan however that just sit between the fan and the case.
I do know DEMCiflex do entire case fan filters but then are on the pricey side not sure if they have done one for the 900d though.