Trimmed mine after trying to convince myself it was comfortable enough. I applied 303 as directed, wiping off any excess then stretched the neck over a glass bowl inserted from the inside. The gaskets have a series of concentric lines molded into them (at least mine did). staying just above the first line, I cut the gasket with a sharp razor blade. I arranged the work so that I could cut all the way around the neck without lifting the blade. The idea being not to create a nick or new cut line that could be the start of a tear. It took a few minutes to get all set up but it went smoothly. I find that as the gasket warms it loosens slightly so only cut down one ring to start, then give it a try. You can always cut another later. Good luck and relaxed breathing- Dana