For whatever reason there's some big gaps in the rear end of the Millennium Falcon
directly above and below the hyperdrive.
If you go to this thread at Eurobricks, check out the posts by "sed6". He did a really good job illustrating this issue and how he fixed it. My hat's off to him. My effort will not turn out as elegantly as his. If there's ever a how-to posted I'll of course attempt it. In his mod he removed the hinge pieces from the ventral crescent-shaped panel. When I did that I was unable to re-mount the panel. The hinges were needed in my case as I needed the pieces at the ends to bend up so they'd secure in place.
I think what I've done helped a little... If you remove the dorsal port side aft panel and flip it over you'll see four tan boat studs. Just remove the two toward the bottom. That will solve one area. There are no pieces to remove on the dorsal starboard side aft panel.
Next remove the ventral crescent-shaped panel. Just pull it straight back and it will come off. I flipped it over and modified it as in my pictures. This is what worked best for me. I removed the black boat studs and the two dark bluish gray 2 x 2s, and then reinforced the whole area between the hinges with three additional 1 x 6 plates in light bluish gray as the hinges cause problems with reassembly (they kept falling apart). It's not easy putting this back in without other parts of the ship falling off. In fact this mod has put me off of wanting to develop the underside of the ship to get a studs down look to it.
I then removed one 1 x 2 light bluish gray tile on the upper right part of the hyperdirve as the starboard dorsal aft hull plate was hitting it causing it to elevate. I then added a bunch of 1 x 2 plates in light bluish gray below the hyperdrive to fill in the gaps between the crescent-shaped panel. I think it turned out decent. I also flipped the two dark bluish gray sloped pieces, one on each side, that serve to hold up the ends so they don't sag. With the flat edge of the slope facing up I felt it put too much tension on the ends, and the trim panel that goes around the landing gear tended to pop off (on the port side only). It can also cause the ends to split if you put too much stress on it.
Click to enlarge the photos below.