Drill the Lower Case
First, you have 10 small holes to drill in pairs along the lower outside of the bottom part of the case. In the photo you can see the closer ones as little black circles and their pairs as little specs of white showing through the other side.
Then you need three larger holes for your bolts running along the bottom surface. Put them wherever you have the room, but be sure that there is one in the front, middle, and back of the bottom.