Have you tried to sand the surface before glueing? It is like mountain climbing. The smoother the mountain, the harder it is for you to grip. Put some ridges into that smooth resin and the metal piece for the glue to grab. Just clean it with alcohol before glueing. This includes adding another layer of resin onto resin. Ideally, add a resin layer before the previous sets so the resin can permeate the previous layer. Truly becoming one piece instead of two pieces glued together. However, those posts would be the most stable embedded into the resin piece. As for which glue, would give you the strongest bond? 2 part epoxy resin glue.
