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I just blow them over with rattlecan clear acrylic lacquer. Make sure the paper is soundly atached, and re-glue any loose corners before lacquering. Spray it in several light coats - don't be tempted to spray a really heavy first coat, as the lacquer can penetrate right thru' the paper and cause unsightly dark spots. I've made 130 plus CBGs and counting, so it does work!
Is polyurethane an acryllic? I recently wrecked a pretty box spraying with poly.
Try painting layers of PVA GLUE on your box,it dries see through and makes the box stronger.
try it out on a spare piece of paper first,so you can see what happens,