I got a couple of these 38mm screw caps with holes for use with 1 gallon jugs. The problem is when I use them with 3-piece airlocks, there's not a tight seal because both the cap and airlock are made of hard plastic. There's no give to either part, so the airlock can't be wedged in properly. Am I missing something to create an airtight seal? I don't want to have to get #6 stoppers because I'd like to smell my fermenting cider and not the rubber.