I cut a slot in the barrel for that rim to set in. I cut it deep so my Plumber's GOOP has somewhere to sit.
Its important that you use a flexible glue that will be able to flex with use, and flex with temp and humidity changes with the difference in your barrel material and the rubber bellow.
Its also important to note that the slot will need to fit to each individual bellow. If you compare several bellows, you will notiice there is a slight difference in the pour used in each one. If you leave too much room and don't make a smooth transition on your bellow, you will see a gap, if you don't leave enough room, you will notice a bulge where your end of your bellow is connected to your barrel. This bulge is VERY bad, because it will allow your glue to give out easily.
Hope this helps......Ask questions if anything is confusing.
Parker