Looks Good. I know what you mean about the sagging. What I have found is that the sagging comes from the back wall of the big rectangle that you attach that assembly to. I made a mental note to back that area with something stiff next time I build it. Here's a picture to try and describe it. Sorry, I'm no artist, lol
backing by
SEBRET, on Flickr
This images was very clear. I glued posterboard throught the "middle hull for strength....but it is not completely flush with the engine section. To prevent the "bowing" you nicely illustrated, I will glue 90 degree cardstock "tabs" around the engine mount to the middle hull. Not sure if I'm making sense, but I will take images after the weekend.
I'm also curious how you dealt with the back of the bridge. I found that when i went to put it around the raised area it sits on, the opening wasn't long enough and i had to cut into the hull a bit. Not sure if you'll understand that but wanted to see if you had the same issue.
One more thing. Not sure if you saw in my photos, but a made a simple addon to keep the back half of the ship level with the front. It attaches right below where the engine block meets the body. Let me know if your interested and ill send it to you.