Nice lookin new windows. What are the yellow things lying at the right of the pic and what do you use them for then ?
Can you do a quick sketch of the truss form above your ceiling . I am assuming that the external shape follows the line of the angle at the edge of the ceiling upwards to an apex. Where are the stays in the truss form. Andy's idea might be easier than you think.
I stand by my previous $0.02 regarding traps in the corners and i think that that may also be accomplished without fuss or ££ or $$ but I will be doing a web rummage as usual see what I can get.
Are you planning to use 1/2 " sheeting ?
Mac