When fins are large or multi-part, take the step of covering them with typing paper and then thin CA. Make them much stronger with small amount of weight. Because the Estes tubes are thin, Instead of all elastic, I use a length of kevlar until just outside the body tube, then attach the elastic there. Where the kevlar hits the body tube, I put several wraps of 1/2" wide tape to make a fat, round ball that will prevent zippering. If the centering rings concern you, just reinforce them -- Apogee has an article on strengthening cardboard rings. In most cases, making changes to increase your surface area for bonding can give you added strength without the weight of plywood and epoxy. I'm working on a LOC Warlock that will be built with foam core centering rings and a 54mm motor.