Tony,
One thing you need to know about plastic cement (tube type) is that it is notoriously fickle stuff. Lay it on thick, and a piece can sometimes take up to a year to fully cure (when all the VOC's evaporate off). It's plastic suspended in a solvent base, and is intended to chemically melt both pieces, and allow them to mix together before evaporating off leaving the joined parts. Since the paper isn't affected by the solvents, the idea is that the "hairs" of paper will become entrapped in the solidified plastic, and anchor it. Problem is, that joint is pretty weak. Even with plastic to plastic joints, the stuff isn't that good, and I've had a number of plastic models simply fall apart after a while back when I was using tube cement to glue them together.
A better method of permanently joining plastic to paper is to first scratch up the plastic shoulder to give it some texture, as smooth sides don't give glues and cements anything to hold onto. For solvent based adhesives, it also increases the surface area that the solvents can react with (creating a better bond). Then using epoxy applied to the inside of the tube (and not the outside of the coupler), slide the coupler in. A few small holes drilled in the coupler can be used to create epoxy rivets, giving further strength to the joint. The epoxy will soak into the paper more, and then when it catalyzes it is fully integrated within the matrix of the paper.
Another method is just to add masking tape to the plastic coupler's shoulder, and simply friction fit the parts together.
