You don't want air from inside the cowl. It usually contains some CO. Exhaust systems are never 100% tight.
You could bypass the heat muff on the exhaust so that the air comes directly to the heater outlet, but that presents another hazard. The exhaust component that provides cabin heat NEEDS air moving around it all the time to cool it. The heat muff prevents engine cooling air from washing over it and it will overheat and burn out without the air movement. Now you have real problems: lots of CO, possibility of fire, and a bunch of expense. So the heater is working all the time, with the warmed air being dumped overboard when the cabin heat is turned off. Your valve needs adjustment or fixing. Or the hose to it could be disconnected and restrained so it still dumps the hot air out the bottom. Legal? Dunno.