Use sketchup's area tool to find the area of odd shapes and then copy the number it gives and multiply by whatever length you need (make sure to divide by 12 to get back to cubic feet).
And by the "area tool", I mean to right click on some panel and then select area--> selection from the menu. A pop up will give you a number in ft^2. Copy that, put it into a calculator (google works great) and then you just add the different sections.