Hi
My modeling history is a bit intermittent and I started trying to make this over a year ago and gave up because it was too difficult.

I'm not very good at modeling and think I lack some basic understanding if the underlying concepts but I have learned a little. I have been trying to make a BMW R1200RT petrol tank and made the front end from a cube by cutting away sections and then using filet to round the edges. I encountered problems when I decided to cutaway the rear section and form the indents by patching pieces together (yellow in the picture). I think that there must be an elegant solution to this puzzle but despite trying various ways this is the best I could come up with.

Hoping that someone knows a simple solution so I can stop thinking about it.