A lot of folks use an old centerfire case. Drill a hole through an extra bottle cap to fit a rimed case, like maybe a 30-30. Open up the primer hole in case and epoxy the case to the cap. You will want to find a stopper for your new spout. Something like a foam ear plug could work. I’ve have also seen the business end of a toothpaste tube or similar cut off and epoxied to a cap.