I used images of the Falcons under side to get some idea of what it should look like. Since the YT-1000 is smaller that the Falcon, I couldn't just copy parts from the Falcon and stick it on the YT-1000. Since I don't have drawings for the bottom side, and I don't think they exist, I can make it up as I like.
I made this diagram to show the difference in the size of the two ships. I used the docking rings or escape pods as they are sometimes called as a point of reference. Assuming they would be the same size on both ships. The YT-1300 on the left and YT-1000 on the right. Other parts like the turret windows are also the same size, but the whole ship is obviously smaller.
Started out with a card stock mockup. This was pretty tricky as the hull is curved and the bottom of the new part had to be in the same plane as the existing center pod.
Once the card stock mockup was acceptable, I just traced the cards onto styrene.
The other side was just a mirror image of the first. I traced the parts from the right to make the left.
Here you can see what I mean about the pods needing to be on the same plane.
The holes are cut for the landing gear and transferred to the hull to be cut out as well.
A box is built for each pod. As you can see it would protrude right into the hull.
Hole marked and corners drilled.