This is a good idea, actually, I think this is how the model should have been (regarding the "wings"). It wouldn't fit the paper format, I guess...
Why don't you do the bottom part similarly as the top? Glue it together, strenghten to required thickness, glue to the top and then glue the strip on the edge?
And yes, this is a big one even in original scale. Here is mine, spread across the A4 workpad

And yes, without the cannons and stuff on the top, it looks a lot like a flatfish
Correct belly would be a challenge - I have found no pictures of the belly, as far as I remember. But perhaps starting by canibalising the landing skids from an X-Wing and continuing on... But then again - there is no room for the skids, because the floor is so close to the belly...
It is a funny little thing - (human) pilots don't fit, it does not land (normally, at least), it has no belly... sign1