I have designed the cab roof and much of the cab interior to be a separate assembly, that can be secured by a series of screws.  As can be seen below, there are two screws at the rear that locate into a tool box that sits on where the bunker projects into the rear of the cab.  As the screws are somewhat lost in the bunker, I have come up with a little dodge where these are retained by an initial nut that traps them in place but still allows them to twist and thus engage in the cab roof assembly.  The other screw comes through the top of the boiler, just inside the backhead.

The roof is connected to these fixing points with some inner liners to the cabs which can be seen here; the nuts for the rear piece are hidden in the toolbox and to the front within the false top to the boiler.  You can just rebate in the rear spectacle plate that will take the glazing material.

The actual cab roof has a double skin, to aid its strength, include the lamp irons and also to assist with locating it on the cab.  The outer skin includes the ribs that appear on the real roof, including a grove to allow brass wire to be used to form the seam to this.  To the perimeter of this, there is a valance.

And this is what it looks like on.  I find that I just can’t make roofs sufficiently well to sit perfectly on the body and nothing shouts “its a model” more than gaps where there shouldn’t be any – be this under buildings, roofs or between parts that have to be joined to structurally stand up!  This is my solution, which I have used on other builds that I have done but it is so much easier when it is designed in.