Twentyone. I tried the blue foam but it was sort of a hassle to cut. I've had better luck with furnace *register* filters (WEB brand Vent Filters). I got a pack of 12 sheets at Home Depot for $3.47. Its exactly half the thickness you need, so cut as wide as the cart opening, fold it in two, then stick the fold into the cart and trim with scissors. Works like a charm!