Apologies, ignore the previous post. I thought you were referring to a PiDP-11.
For the PiDP-8, more to the point: what you describe is 'normal' (as in, poor design from me). Originally, I intended the panel to be fixed with the two 30mm screws, but that is a bit ugly. So nobody did it, relying on friction-fitting the panel instead.
The fix for wobbliness is a bit of black tape (or black paper for extra width) inside the inner ledge of the case, to friction-fit the panel into the case. It sounds amateurish (it is), but proved very effective, more so than some experiments I did with magnets etc. The Building Instructions page gives some more detail.