Keep it retro? - what could be more retro than a hand formed alloy dome, as fitted to hundreds of buses, coaches and commercial vehicles from the1930's to the 1960's? The basic shape for the one I used came from Dennis Poore's 1940 Dodge VK62B crewbus, and they don't come much more retro than that!
Oh well, there's no accounting for taste I suppose, so flat top it is:-
Best Wishes, DW