a simple bucket, filled with zeolite, with a pinprick hole and under it under the stand holding the bucket, a water container with a large funnel with some cotton stuffed in the neck will do the trick

most filters also have carbon which removes the chlorine gas that forms out of the sodium hypochlorite they also add. that stuff is nasty too... it does eventually gas off and the zeolite also removes the sodium hypochlorite, it additionally removes the calcium, magnesium, iron and other shit in the water (much like RO but much much cheaper, i mean like $50 is gonna filter you like 10,000 gallons)

zeolite isn't that expensive though, it's just a relatively common form of stone