I agree with Whatnot but I suppose there's no harm in experimenting to satisfy your curiosity, and I can certainly understand why you would ask the question. If I attempted that I'd probably utilize a tightly compacted chunk of aluminum foil so that it would stay in position when wedged inside the upper portion of the chute.
My
opinion is these smoker manufacturers either a) account for the small amount of exhaust from the lid and grease chutes and tune their main exhaust sizes accordingly (and/or adjust the fan speed) or b) the additional exhaust is so insignificant that it need not be accounted for. It also
seems to me that some leaks like this in a forced air smoker are of little consequence compared to the adverse effect in--say--an offset stick burner smoker that relies solely on intake and exhaust. But I'm not a smoker designer nor am I an Engineer, so my opinion means little. But I would ask myself how I would truly measure the results of tweaks like this.
Merry Christmas!