Illini's got it, I think, as re: two cemented caps. My own self, if it were possible, one cap with a fitting to draw a vacuum in the entire tube, with a rubber flap on the underside, or the fitting filled with a solid rubber plug and the vacuum drawn via some large bore needle. Coat the whole cap with PVC cement once the vacuum is drawn and jam on the next sized larger cap. A really sticky silicone sealer around the cap's seam with the pipe would fill the bill. Opening day, cut away the sealer and what ever form of cement solvent, or a chunk of hacksaw to cut off the cap. Inelegant but workable.