I agree with gbolton about the hollow core vs. solid core door. I spent years trying to get a hollow core door to work. Ifinally gave up and bought a solid core. I've measured it and it's within 1/32 in. of dead flat. I use a piece of ceiling tile from Home Depot taped down to the door for a glueing surface. (Back side up). Then use the door surface for assembly. I put paint masking paper on the ceiling tile and door surface for protection,and throw it away after each project. Works good for me.

pmw