After letting your tubes fully dry for one day, come back to the project and make a cookie out of non-toxic clay.
Use the tube like you would use a cookie cutter, but leave the clay "cookie" in the end of the tube's hole. Once the clay dries, the plugged end will work as the tube's "back door" for the mason bee.
(If you don't plug the back, the bees will spend their time doing it before they start to build the pollen plug.)