I can try... it's been quite some time since I worked on this.

The highlighted areas where what I constructed. I believe I started out with 3.2mm square plastic tubing along the entire length of the ship. At the rear, near the engines, I put some pieces of round plasticard rods on either side to fill the space. I took the 3.2mm square and put two pieces side by side and glued them together for the front with a piece of half round rod to cover the gap. cut these off at an angle and filled the hole with putty. made another little bit to taper it into the singe piece and provide a place for the metal rod that runs the length of the single square piece. did a third layer of the 3.2mm square only up by the front of the craft and used some thin plasticard strip for detailing it and the other areas on the craft to match the feel of the rest of the lander. Made a few shims on either side to fill the space where the lander's clamp arms meet the body. The last thing was a few plasticard bits on the rear underside to fill the obnoxious hole by the landing gear and to make a couple of turbine bits to sit in the engine inlets.
I think that's everything, hope it helps!