I don't even cap the bottom. I just double wrap with saran wrap and use rubber bands and some shipping tape to close the one end. Put the pipe into one of my plastic juice pitchers that I use for lye and wrap the part that's in the pitcher with a junky old towel. I leave the top portion unwrapped after pouring in the soap batter. I don't want a partial gel so I try to encourage it with the towel. That being said and after many years of fighting with the soap made in pvc, I gave up and bought some easier to use cylindrical molds from ED. They do have caps at the end and I do use them