Someone has recently made a display extender that could be used as a basis for a removeable display idea where the main unit is more securely fixed.

That could certainly be done, but seems a lot more complicated than necessary, and you still have to carry part of your car around when you leave it; I went the route of making it look like there's nothing to steal when the player isn't running. Serial VFDs are (compared to an Empeg) cheap and plentiful; if someone wants to steal an innocent looking cubby out of my car they can go ahead.. The good stuff is (relatively) safely stored in the trunk (which is where I'd usually leave the player anyway, only now nobody sees me moving something shiny out of the dash and into the trunk).