You could just use the guts from any household furnace or airconditioner filter cut down to fit. There is also a trap door just under the cowl for really cold weather ops but be careful if operating is powder snow as it can get inside the engine compartment through spots like the exhaust hole in the lower tub and worn out/beat up steering arm bellows. The powdery snow can get into the carbs and ice them up as well as get sucked into the recoil start mechanism. Ask me how I know these things.....The symptoms on the recoil are no pull start, the rope just pulls out with no engine rotation. Eventually the heat of the engine after shutdown will restore it but in the meantime it's good to know the "broken pull rope" method of starting a sled. Hint: the Indy's came with an emergency "start strap" stock.