[IanW]

It's been a while since I've used SolidWorks (you use Inventor when you intern at Autodesk I guess), so this may not be the most efficient way.

Bellypan:
I make four holes in the corner of the bellypan - two triangles and two squares - and then use patterns and mirrors to complete the entire cheese hole pattern. If I want to and holes/extra webbing for gearbox cutouts/electronics, I go back and make them separately by cutting away the material I don't want and then extruding the shapes I do want back in.

Gearbox:
With a gearbox it's just a slow process of outlining the areas that will remain (around bearings, spacers, bolt holes, etc.), followed by adding in the lines that will connect them. Attached is a picture of the drawing I created for the lightening pattern I used for a gearbox project I've been working on. Notice how I used fairly few dimensions for the amount of sketch geometry. I did this by using a lot of sketch constraints, ultimately making the sketch easy to use if I ever want to go back and make changes.

Hope this helped, I'd gladly try to answer other questions.