I cant find small coolers with a hole....and I dont wanna drill a hole in a cooler for the tap. Now my only option is to buy a 15 gallon cooler to use as a mash-tun, but I wont brew more than 5 gallon in it. Will the empty head-space affect my wort? Will the water temperature drop in it because of it (without maintaining a constant temperature for 60 mins)?