John,
Might I suggest a call to a local glass merchants for an 18" x 12" slab of plate glass (at least 1/4" thick)? it really makes assembly of the sides and roofs of Slater's carriages a breeze...
My Maunsell carriages use an inverted 'T' shape to strengthen the roofs and I build down from there, splitting the body and solebars. Your carriages are going to be stronger at the top of the body (the Maunsell carriages have their sidelights extending almost to the cantrail), so you may get away with a separate roof.
Just a thought...
Steph