Hi Dave, yes I found the 2 mm a bit too bulky and difficult to work with for the wheelhouse itself, funnily enough I was looking at the real boat in Milford and thinking about the mast, after all it is only supported by the thin side frames of the windows.
I haven't started the top windows yet, (been putting them off because of the fiddly aspect), but I will probably put two cross struts in of 2mm just under the mast going side to side just for extra support, nobody will see them unless they get on hands and knees. They will be as deep as the slope on the top reverse windows. This will also help the wheelhouse keep its shape.
I have been using the plumbers solvent, the type they use for cementing plastic waste pipes together, i find it easier to use and stronger than the watery stuff which evaporates in 2 seconds, and a lot cheaper.