I need to make a TD Garden Stadium Cake - a couple of questions

I am making a cake from the attached picture (pic called stadium to use) to feed 30 people. With a basketball on top... see second pic for example of overall cake (pic called cake to copy).

The cake shouldn't be to hard in theory.  The main portion is a square cake.  I'm thinking 2 9" squares (2" high each), which feeds 27 people. 

Then above that a 8" square on top of that - allowing it to mound in the baking process.

I am concerned about the roof.  It's juts out from the building.  I know I can make it from gumpaste and dry it now and it will retain its shape.  (The cake is due in exactly 1 week).

I would also make the thing that juts out of the top (not a chimney but looks like one) from gumpaste.

Every one agreed?  Anyone want to give me a better way to go?  I'm all ears!!

Oh - and to anyone who has seen this building - what shade of yellow is that roof?  It looks kind of French's Mustard to me - but it's in the dark.  I've never been to this stadium.  Help - please!!