My more beat-up Gridcase 1520 was missing its battery eject button. To get the battery or power supply out I needed to poke a flat-bladed screwdriver into the hole where the button should be. Very inconvenient!
Since I had my other 1520 disassembled I removed the battery eject button to have a look at it. It's shaped like a long, thin 'T' with a step in the middle to keep the button held in place against the rear of the battery compartment. The 'base' of the T is sloped so that when the button is pressed it slides over and depresses the spring-loaded clip that holds the battery in place.
I measured it up and managed to bash together a workable model using OpenSCAD. A friend with a 3D printer very kindly printed a couple for me. It fits almost perfectly!
The OpenSCAD source and STL model are available for download.