The only place to vent your cooktop is outdoors. You want the shortest run possible. DO NOT vent into any other space. Mold, mildew and grease will all be problems. If you can't vent through the sidewall goe through the roof and install a roof mounted damper. Keep in mind your vent manf. will sate the maximum length duct you can use. Use only metal duct, no flex duct here.